Most people who play Pathfinder do so because they played Dungeons and Dragons 3.5. They are, after all, the primary demographic which Pathfinder has been geared towards. Paizo created Pathfinder with the intent of carrying on 3.5's legacy by continuing to provide compatible products to the fans of that game after it was discontinued. However, the more I watch Paizo, the clearer it becomes that they are an adaptable and forward thinking company. They understand their customers, and their market, and know how to leverage their resources. An excellent example of this is the Pathfinder Beginner's Box, which has been universally hailed as the best starter box-set since the original red box.
So it strikes me that if Pathfinder has created such an excellent product for getting new people to play their game, then it is reasonable to assume that there are new people playing it. Those new players, by definition, have not played any previous RPGs, such as D&D 3.5. Of course we don't actually know how many new people have been turned on to our hobby through the Pathfinder Beginner Box, but it is safe to assume that there are some, and that there will be more. As these players connect with the hobby, they'll move on to the Pathfinder Core Rulebook and Pathfinder Bestiary for a more complete version of the basic rules. And then they'll want more, and they'll turn to supplements.
Now, Pathfinder has many fine supplements. The Game Mastery Guide is a particular favorite of mine. But one of Pathfinder's great strengths is its ability to draw on any of the supplements of D&D 3.5. I fear that new players and game masters may not be aware of this treasure trove of books just waiting to be used in their games. And so I've compiled a list of Dungeons and Dragons supplements (mostly 3.5 but some are D&D 3.0) which I feel work best with the Pathfinder role playing game. This list is by no means complete. The list details a modest selection of books which I am familiar with from my own collection. I still don't own about 1/4 of the official supplements, and the list below is not complete even for the books I do own. Only the ones which are truly excellent.
Fiendish Codices: I & II
In my most humble opinion, the two Fiendish Codices were the best books released for D&D 3.5. The first codex, Hordes of the Abyss, details demons. In D&D (as well as in Pathfinder,) Demons are being of pure chaos and evil. The second, Tyrants of the Nine Hells, details devils. Again, both D&D and Pathfinder use the word "devils" to describe beings of pure Law and evil. Demons and devils loathe one another, and according to D&D lore, have been engaged in a conflict called "The Blood War" since the dawn of time. Both books begin with a chapter expanding on the 'fluff,' (or 'lore,') of these evil creatures. This is something which I found considerably lacking throughout all of D&D 3.5's run: good sources for information which isn't strictly mechanical. I particularly enjoyed the story of The Pact Primeval which TotNH begins with. The books go on to tour various locals in both the Abyss and the Nine Hells. Both also contain information on the Lords and Ladies of these dark places. Rulers of incalculable power and evil, many of which are so fascinating that I had a dozen campaign ideas for them before I finished reading their descriptions. Aside from the above, each book contains new feats, spells, prestige classes, and monsters. Most of which should be compatible with Pathfinder.
Many of the characters and locals in these books are protected intellectual properties of Wizards of the Coast, so Pathfinder is unable to make use of them. Pathfinder's world of Golarion has done a great job making up for the loss of the traditional D&D cosmology and history, but it simply doesn't have the same impact for me. Maybe I'm simply not familiar enough with Paizo's game world, but I can't bring myself to abandon the gods, heroes, and villains which I came to love in my early days of role playing. But even if you're perfectly happy with Golarion's denizens, these two books are worth getting your hands on.
Races of Stone, Destiny, and The Wild
As you can easily infer from these book's titles, they provide more detailed information on the basic races of Dungeons and Dragons. Out of the seven core races, each book provides an entire chapter devoted to two of them (save Destiny, which covers 3). Each book also introduces a new playable race, given a similar amount of detail, which holds to a common theme. Races of Stone details Dwarfs, Gnomes, and a large, hard-skinned race called Goliaths. Races of the Wild covers elves, halflings, and a flighted race called Raptorans. Finally, Races of Destiny goes over Humans, devotes on chapter to both Half-Elves and Half-Orcs, and includes a new race called Illumians, which are the living embodiment of language.
The racial chapters are a good hefty size, between twenty and thirty pages in length. Each race is dissected in detail, from their psychology, to their common grooming practices. Artistry, folklore, religion, the list goes on! These chapters have proven invaluable to me over the years, and to this day I still grab these books for reference if I'm including an important NPC of a race I haven't used in awhile. And on a personal note, I really love Illumians. So much so that I included them prominently in The Girl and the Granite Throne.
I've mentioned before how great this book is. In fact, I wrote an extensive post detailing my reasons for using one of its alternate rules. That entire post was based on two and a half pages of this 218 page book. I won't say that everything in here is gold, some of the ideas presented in it are actually quite bad. And many of interesting or good ideas presented in this book have actually been updated and reprinted in official Pathfinder supplements. However, there's still a lot in here for Pathfinder players to enjoy. Environmental races such as aquatic dwarves or arctic elves; bloodline templates which allow players to gain minor--or major--abilities due to a special ancestry; paragon classes which allow characters to level up as a model of the traits their race embodies; and that's all in chapter 1!
I can't think of any book which I would recommend to new GMs more highly than Unearthed Arcana, because it does a good job of teaching GMs how to apply rule 0. The core rulebooks of an RPG always throw in a few lines to the effect of "But if you don't like it, change it! It's your game, you can do anything you want!" It characterizes rule 0 as a blunt instrument, requiring no forethought. By presenting balanced alternatives to the official rules, Unearthed Arcana provides a model of what house rules should look like.
Epic Level Handbook
In the first edition Dungeons and Dragons rulebook "Men and Magic," Gary Gygax writes that there is no limit to the number of times a player could theoretically level up. First edition modules were often marked "An Adventure for Characters levels 28-32," or even higher than that! The 3rd edition of Dungeons and Dragons is a little more reserved regarding levels beyond twentieth. Given the massive scaling differences between the two, I can't blame them. Yet as a player and as a GM, I've always liked the idea of a character being able to scale infinitely. I would love to see my players bite and claw their way through forty or fifty levels, eventually growing powerful enough to replace a god. And once that happened, they could become a permanent part of that game-world's pantheon.
Even if you're less inclined towards allowing deicide, or similarly grandiose feats in your game. the Epic Level Handbook is a severely underrated guide to running games beyond 20th level. It includes a number of tools for GMs to help them create epic level obstacles and epic level monsters which their players must face in order to accomplish the epic level goals the book suggests, in order to win some epic level loot and rewards.
Manual of the Planes
As I mentioned above, I'm not intimately familiar with the world of Golarion. Thus far I have stuck to the classic Dungeons and Dragons flavor whenever I'm not using something from my own campaign settings. So I don't know what offerings Paizo has with regards to planar adventuring, but I have a hard time believing it could be much better than this.
I've honestly read the Manual of the Planes cover to cover a number of times for the sheer pleasure of it. And truth be told, I've never had the opportunity to run a game where my players spent a significant amount of time off of the material plane--but ever since I read this book I've been looking for an excuse to send my players out into the multiverse to explore.
This book is a spark to my imagination. What kind of adventures might my players have on the Twin Paradise of Bytopia; a plane with two landmasses facing one another, each serving as the sky of the other. Or what about the supremely lawful Clockwork Nirvana of Mechanus, where gravity is dependent on which miles-wide cog you happen to be standing on. What about the Infernal Battlefield of Acheron, where dead warriors fight on in an unending battle which will last for a hellish eternity? The possibilities seem endless. Such places could be the location of a single whimsical adventure, or an entire deviant campaign.
The Books of Exalted Deeds,
and Vile Darkness
Named for the artifacts of the same name, these opposing tomes describe the absolute pinnacle of all that is good and holy, and the darkest depths of all that is depraved and profane. To my knowledge they are the only books which were sold with stickers bearing warnings of mature content, not for players under 18 years of age.
Both books are excellent examples of what good and evil should be in the game. BoED is essentially a long-form version of my recent post on Paladin Overzealousness. It encourages GMs to provide paladins (or non-paladin characters who wish to hold to an 'exalted' code of ethics) with legitimate moral quandaries. The book also stresses that these moral quandaries should be solvable without forcing a character to betray their ideals, and it provides tools to help GMs do that. The Book of Vile Darkness is, not surprisingly, just the opposite. Aside from terrifying monsters of pure evil, spells which cause unnecessary suffering, and basic rules for torture, the book includes a lot of information to help GMs build better villains. In particular I liked the section near the start which presented a number of simple villain archetypes.
As I mentioned above, this list is not exhaustive. Wizards of the Coast released an immense number of supplements during the run of Dungeons and Dragons 3.x. Books like Exemplars of Evil, or The Stronghold Builder's Guidebook still see use in my games. But in going through my own collection, these are the books which really stood out to me as having the most impact on my play over the years I've had them. I would recommend any, or all of them, to a Pathfinder player looking to expand their collection.