There are a lot of sites out there that recommend books. They are a great starting point, but they do not always list by the first book in the series. Also, some of the suggested books are of higher maturity levels, so you'll still want to do a bit of research on your own before choosing a book from one of these sites. Here are a few I've found that are pretty good:
http://www.bookspaceforschools.co.uk/bookflavour/ -
This is coming soon. It looks really interesting.
http://www.makeuseof.com/tag/7-sites-decide-book-read/- This is an article of 7 recommended sites for choosing your next book.
http://www.scholastic.com/bookwizard/ -
Scholastic will sort books by reading level.