Here's a list of books that are particularly useful to fantasy and science fiction writers.
A dictionary.
Certainly nowadays there's online versions of dictionaries, some with an ample wealth of definitions, but I'll agree that there's something about the word lists supplied by a dictionary that are evocative and imagination sparking. You can even use dictionaries for writing exercises. A thesaurus can be similarly handy, but beware the pitfalls of using a thesaurus to vary your word choices when writing. If you do not know a word’s precise meaning, a thesaurus’s baleful influence can often lead you astray.
Books of specialized knowledge.
Books of specialized knowledge are always intriguing, whether they're technical dictionaries, lists of ancient machine parts, or anatomical lore. Looking in bargain bins at used bookstores can often yield a treasure trove of inspiration, and nothing gives a story quite so much authenticity as specialized lingo. Luc Reid has written a nifty little book, called Talk the Talk, that lists vocabularies for a number of sub-cultures, from goths to white-water rafters.
History texts.
Historical accounts are always intriguing, and also yield the kind of precise detail and worldbuilding that specialized lingo creates. Find the historical journals that match the time era you're writing in and look for the details that create authenticity: the number of loaves of bread an average court would go through in a week, for instance, or how spoiled meat was cooked in order to best disguise its taste.
Mythology texts.
Whether they're western classics such as Malory's Morte D'Arthur or Edith Hamilton's account of Greek and Roman mythology or lesser-known mythos, such as Jewish lore or Hindu mythology, such texts provide the sorts of stories that fantasy and science fiction writers tell over and over again. Read through the stories and look at your own narratives to see where they coincide and where you might draw on the mythology to enrich and deepen your own story. Try telling one of the stories from the point of view of one of the minor characters, or updating the story so it fits within some modern -- or even future -- scenario.
Art.
Art books, whether they contain abstract art or realistic pictures, can act as inspiration and spark descriptions that are lyrical, precise, and evocative. Spend some time looking to find out what pictures speak to you - which are the ones you might hang onto a wall or choose to keep in your wallet? Are there any that disgust and repel you? Figure out what creates the emotion and try to create the same effect verbally, either by describing the art or by trying to evoke it through your choice of words and images.
