I know a huge challenge for families is finding books that match the reading leveling system we use here at school during the summer. I have two websites that should help with that challenge.
http://home.comcast.net/~ngiansante/
This website has lists of books by grade level, author and title. If you look at the grade level lists, the Guided Reading Level is located right after the title.
http://minibooks.scholastic.com/minibooks/home/
This is a subscription site from Scholastic. I believe it's between $20 to $30 for a year. If you haven't checked out the parent section of the website please do. They have free printable, activity ideas and articles. It's a pretty good site.
I will continue to post ideas for summer reading and other subjects as I run across them.