The Negative Trait Thesaurus: A Writer's Guide to Character Flaws - Angela Ackerman, Becca Puglisi

Excellent reference book for writers. It helped me sort out a very serious case of writer's block. The tips on how to weave flaws into the characters, what to look out for to make sure that your flawed character doesn't turn into someone completely unlikeable and many others. It's a must-have in any writer's arsenal of reference books.