The replies veer off into writing books, of which Stephen King's has been recommended the most often. Personally, I like William Zinser's On Writing Well, and several others of his books, far better.
But Nathan himself said it was The Great Gatsby. I suppose nothing is more inspiring than a great novel you can return to year after year, seeing new layers of meaning each time. People who teach great literature have this experience, and writers can too if they reread great books every so often. Others suggested The Bible (diversity of plots and characters), The Complete Works of Shakespeare (same but also the writing), The Sun Also Rises, Of Mice and Men. I would consider Little Women, Mrs. Dalloway, Pride and Prejudice but probably would choose a book of Chekhov's short stories.