Here’s a definitely incomplete list of books and websites I’ve found useful for research and inspiration.
Castiglione, Baldassare. The Book of the Courtier.
A guidebook on how to behave in an Italian Renaissance court, written in the early sixteenth century. Also applicable to worldbuilding for early modern fantasy settings.
Edelstein, Linda. The Writer’s Guide to Character Traits, 2nd ed. Writer’s Digest Books, 2006.
Written by a practicing psychologist, this book summarizes research of use to a fiction writer regarding human personality and behavior. [Caveat: the section on transgender people in the chapter about sex gets several things seriously wrong.]
Genta, Giancarlo, and Michael Rycroft. Space: the Final Frontier? Cambridge University Press, 2003.
Explains past (and possible future) space travel technologies, and explores the practicalities, benefits and dangers, and philosophical and ethical issues of space exploration and human settlement of the solar system and beyond.
Machiavelli, Niccolo. The Prince.
Excellent inspiration for your more scheming and power-hungry characters, regardless of genre and setting.
Gies, Frances & Joseph. Daily Life in Medieval Times. Barnes & Noble, no date (omnibus reprint of Life in a Medieval Castle, Life in a Medieval Village, and Life in a Medieval City, published in 1969, 1974, and 1990 by HarperCollins).
If you’re writing historical fiction set in medieval Europe, this is obviously useful. It’s also a great starting point for worldbuilding for a medieval Western fantasy setting, though of course a fictional world will diverge from the real world in whatever ways fit the story and your creative vision; see Patricia Wrede’s worldbuilding questions below for more guidance on this.
Roach, Mary. Packing for Mars: The Curious Science of Life in the Void. W.W. Norton, 2010.
Absolutely hilarious book about the awkward practical problems of space travel. Covers everything from astronaut selection and fears of “space madness” to space food, space showers, and space toilets. If you’re writing near-future science fiction set in space, you might want to check this out. (Actually, even if you have no intention of ever doing such a thing, you might want to check it out, because it’s really, really funny. Like Mary Roach’s other books, though, it’s not for the squeamish.)
Sun Tzu. The Art of War.
Ideas for strategies that characters may use in military conflicts – or sometimes other conflicts.
Tunis, Edwin. Weapons: A Pictorial History. Johns Hopkins University Press, 1999.
A brief but useful history of weapons and armor from the Stone Age through World War II, with detailed drawings and clear explanations. Unfortunately only covers Western weaponry.
Wiseman, John “Lofty.” SAS Survival Handbook: How to Survive in the Wild, in Any Climate, on Land or at Sea.
If the awful things you plan to do to your characters include stranding them in woods/mountains/a lifeboat (tiger optional)/the postapocalyptic wasteland, go here first.
Author Patricia C. Wrede’s classic guiding questions for fantasy worldbuilding, first posted on the Internet in 1996.
Sometimes you’d like your characters to achieve their goals through devious tricks. Here’s a famous list of possibilities, derived from ancient Chinese strategic literature. (Caution: TV Tropes is addictive enough that it should probably have a U.S. Surgeon General’s warning label.)
On the other hand, sometimes you want to have a plausible reason why your characters can’t or won’t use what seems to be the obvious solution to a problem. Here are some reasons.
Common linguistic bloopers in amateur (and, unfortunately, sometimes professional) writing.
A useful tool for finding character names, especially for American characters born in the late 19th century or after. Click on “Popular Names by Birth Year” to find the top 1000 names for boys and for girls in the US for any year after 1879.