Since I work in a library (and am a librarian), I usually use Common Sense Media, which contains reviews based on age appropriateness and learning potential, more for movies and apps versus books, but the site has some features that parents (and educators) will find helpful.
For example, when it comes to finding books for my daughter, I normally look at book reviews in publications like School Library Journal -- or better yet, go down the hall and ask Jennifer Schultz, our Youth Services Librarian for recommendations (she always has great picks for us!). However, the thing I like about Common Sense Media's Reviews is that they include a "What parents need to know" chart (see example below), which quickly points out hot buttons like violence, sex and language, etc., giving me fair warning. It also includes educational value, positive role models, etc., areas that I think are sometimes given short shrift when compared to something like bad language.
In the example above, for the Glass Sentence, you can click on the "See full details" link for each area, such as the positive role models and get details such as:
I also like how their user reviews are broken down by what Parents Say and what Kids Say. I usually look at these reviews (especially those written by kids) if I'm undecided.
Bottom line, when my daughter, who is only ten, wants to delve into the latest book aimed at teens (gulp!), I have found Common Sense Media's reviews to be immensely helpful. I hope you will, too!
~ Alison, Electronic Resources Librarian