Well, that's kinda obvious, isn't it? Sure there are a lot of good long reads, but which books are worth keeping in reach and what kind of books do inspire your work?
To me, game guides have proven to be quite handy. They usually describe every bit of game play features of the respective game in a very detailed fashion. Maps, complete item lists, enemy classes, characters, move lists, weapons, secrets and puzzles, all here in print. In general I can recommend the game guides from Brady Games and Prima. Be careful when buying guides from other publishers, sometimes there are not licensed and offer no screenshots or original art, are badly printed and very text heavy.
Also books about ancient mythology, science or history made for children and young adults. They usually contain a lot of images and very brief text blocks. This allows you to quickly browse and judge a lot of content. On a superficial level, you learn about cool stuff, like pirates, greek monsters, ancient warfare, robot technology and such. But because of the colorful presentation you also get a sense of it's entertainment value. You still can dive deeper into the matter with more sophisticated books later.
Of course game design books, art books and comic paperbacks always make good sources of inspiration.
Board game stuff
Card games, spectacular action games, table tops, miniature gaming and things like that manage to provide a decent challenge and spark the players imagination without sound and pixels.
To me they serve as a strong reminder how imagination and playing with friends can make an impact on gameplay experience. Having friends actually in the same room, setting up the game together, taking turns, mouthing sounds, tying to beat each other. Often enough modern games don't cherish this kind of multiplayer feeling and force friends to sit in front of different consoles to play together online.
They also remind me how effective it can be to offer the player a game he can touch. Rolling dice, flipping cards, moving tokens over the board or sticking tiny swords into the pirate's barrel. All that adds to the gaming experience. Nintendo knows that and offered touchable game devices, like the DS and Wiimote
Also fantasy games and card games for example offer a huge huge huge amount of content and serve as an idea library similar to mentioned books.
Toys and action figures
Yeah, I'm a nerd and this might be the hardest recommendation to sell on this list. Action figures to me are not only sculptures of fun and well designed characters, sometimes marvelously executed, they also are playthings. Do you remember, back in the days, the glee you felt, when you pressed the action button on the back of your favorite figure and watched his arms waggle? Do you remember some of the epic epic battles your heroes and villains had?
Action figures and toys are the tools for everybody's epic D.I.Y. adventure and agents of imagination. Many people, when they grow up kinda step away from the power they had when they were kids. They sell their figures on ebay or hide them in the attic and even when they keep owning action figures, they don't take 'em out of the package. Having playthings around, especially ones you yourself actually played with back in the days, helps keeping in reach some of that limitless childlike imagination you once had.
Jason Rohrer's Sleep is Death is exactly that kind of plaything, but in pixels, so we grown up game designers can play with it and don't need to feel childish. ;) Still, to me nothing beats the real thing.
Old skool games
In comparison to today's games, the games I grew up with are rather short, simple and lack impact. But what they lacked in volume, polish and technology, they made up in groundbreaking gameplay ingenuity. Said volume, polish and technology standards can now be met with a rather small team. So knowing how to make awesome games within those limitations makes for great indie games. Old skool games of course also serve as reference for modern pixel artists and chip tune musicians.
Playing old games on an emulator is one thing. But blowing into the cartridge and experiencing classic games in their natural habitat, password system, bad resolution, unforgiving difficulty and all, is something completely different.
In short: Know your roots and experience them firsthand.
Quick sketching tools
Even if you are no game artist, proper tools and skills to express yourself on paper and to quickly note whatever brilliant idea fills your mind right now is essential to the creative process. Quickly sketching maps, items, jumping patterns, enemy types, powerups and all that jazz can really advance any creative exchange with other designers and artists.
Especially when you are not as skilled with drawing as you might like, having good tools, like a wide palette of colors, templates and plotting paper really helps. Oh, and a good camera to not only capture inspirational motifs, but also function as a quick substitute for scanning your sketches, might also be a good idea.
Well, that's some of the stuff I keep around my office to refill my creative batteries, to serve as an inspirational library and to make my ideas flow. All that stuff also looks cooler than a Newton's cradle. ;)