Day 15 of the 40 day book challenge is 'a book that I have read the most number of times'.
Hmm. I'm not sure. The book I've read the most times lately is Catherine Newman's Waiting for Birdy (it's probably the book I've mentioned the most on this blog too).
As a child I probably read Enid Blyton's Malory Towers series the most.
As a teen it was Thinking of You by Jeanette Nobile, which I wrote about for Girls Heart Books.