Tobin's Spirit Guide (Erik Burnham): Recently, a new official Ghostbuster's version of this book came out. Written by Burnham (with the help of Stantz and Spengler of course!) this edition features some of our favorite spirits from the Ghostbuster's franchise. It also includes a variety of other common mythological creatures like household spirits or poltergeists. Each entry features a Ghostbuster's rating of spirit class level and what the spirit may be comprised of or where it is located / anchored. Additionally the entries feature some great bits of real-world mythology. Building upon that the way in which they can be angered or appeased and how dangerous they are. The book is written almost has a field guide, and (barring the lack of stats) is the best bestiary or monster manual you could ask for to run a Ghostbusters campaign. All you would need is a book on the Ghostbuster's gear. Even if that isn't your goal, the ideas you could pull from this book are numerous and inspiration on how to had even more incorporeal danger to your world, immense!
The (Complete) Tolkien Companion (J.E.A. Taylor): My version of this resource is a bit outdated. I don't have the "Complete" in my title. Given that much of the source material still exists as part of Tolkien's work, itself, I don't see a need to get the revision too quickly. Though it is on my list. This book is an encyclopedia, there really is no other way to describe it. My copy is over 500 pages long and there is barely any pictures included at all. Maybe I do need an updated version. Anyway, this is your go-to resource for any questions you have about Middle Earth. Beyond the Lord of the Rings, beyond The Hobbit, and beyond even the Silmarillion, there is a vast fount of knowledge and lore these stories are built upon. Tolkien was a world builder and language developer, and the tales run deep in the land of wizards and hobbits. I know there are other resources you can find out there, but this is probably the most dense there is. If you really want a comprehensive reference to one of the foundation stones to modern fantasy adventure, this is it.
The Hyrule Historia (Eiji Aonuma, Akira Himekawa): Honestly, this is probably my favorite of these three resources. It is as large and dense as the Tolkien Companion, but includes the fun (and more) that you can get from Tobin's Spirit Guide. There are four major sections to this book, divided up amongst specific topics unlike the other two. The first of these is a large section discussing the Skyward Sword, its lore, ideas, and concept art. While amazing, I am disappointed by how much space this single game takes up, but the rest of the book helps make up for it. The second section is my favorite. The Legend of Zelda series has been tackled game by game, with little to no connection between any of them aside from general themes and feeling. Even these deviate for each game making playing the game feel both familiar and unique. For those who have even played a few of the games and completed even less (sadly me), you still come to understand every one of them is connected somehow. This section is a timeline. It orders the eras of the world, describes what happens and when, from the creation all the way through the voyage upon the ocean in Windwaker and a New Hyrule in Spirit Tracks. For the first time there is a timeline of events, and yet still some connecting pieces and lore to be discovered. The third section of the book goes through 25 years of concept artwork. It is fantastic to see how things have changed and how different artists perceived the world. This section provides a ton of inspiration for GMs. Following up on that is a catalogue of the games, what generation they are, and the story they tell. And finally, the last part is a comic for you to enjoy telling a part of the Skyward Sword story.