If you're writing about a book title *within* a sentence, you italicize it. If you're using a book title as a heading or standalone element, you can also use sentence case capitalization (only capitalize the first word and proper nouns) as shown here:
*Correct:* I recently read *To Kill a Mockingbird*.
*Correct:* My favorite book is *Pride and Prejudice*.
*Correct:* *To Kill a Mockingbird* is a classic novel.
*Correct:* To Kill a Mockingbird is a classic novel. (In a heading or citation)
In short: Italicize it, and use either sentence case (only if a heading or stand-alone element) or title case (capitalizing all major words) within a sentence. The italics are the key, however.