Reader Age

Though this article was written about magazine articles, it's also applicable to books.

http://www.institutechildrenslit.com/rx/ws05/agerange.shtml

Boy Readers

They don't just want to see boy protagonists. 

http://kidswriterjfox.blogspot.com/2012/07/guest-post-andy-sherrod-on-boy-readers.html

Endings

Make sure the end of your book is satisfying to your readers. 

http://blog.janicehardy.com/2011/07/its-over-getting-readers-to-ending-and.html