"Writers should consider what point or points in a culture's development they are going to portray. Each has its particular allure and interesting characteristics. Cultures cannot remain static for long and are constantly changing, expanding, and contracting. Is the culture in the ascendant, with dynamic leaders striving to carve a place in a world of dangerous competitors? Is it a strong, powerful state, secure against all but the most dangerous enemies? Is it an ancient, now-decadent culture with indifferent leadership and on the brink of imminent decline? Or has that decline already begun, with collapse from the inside being hastened by aggressive outsiders?" --Michael J. Varhola
The above quote is from The Writer's Complete Fantasy Reference, written by the editors of Writer's Digest. An excellent book, and if you writer epic or high fantasy, I strongly suggest reading it. It literally lists different cultures around the world from which you can base ideas, different sorts of scrying, common fantasy races, types of weapons and armor, and so much more. The Anatomy of a Castle chapter is especially helpful for medieval European-based fantasy.
On a side note, I've finally named my current work in progress: CITY OF TEETH (CoT for simplicity's sake). And, now that the holidays are passed, I shall resume my querying. :D
What books on writing have you found especially helpful?