Derek Morgan
Derek's favorite book is Have A Little Faith by Mitch Alborn.
Aaron Hotchner
Aaron's favorite book is Boxcar Children by Gertrude Chandler Warner.
( This is also Thomas Gibson's favorite book, so why not? )
Spencer Reid
Spencer doesn't really have a favorite book, it's mostly the book that he had just read. However, Alice in Wonderland by Lewis Carroll was really important to him as a little boy.