An interesting discussion is taking place in the blogosphere, regarding selection of books, with an emphasis on when decisions about purchasing is equal to censorship and when it is not. The origin of the discussion is the news story about a school board who decided not to purchase some books from a list prepared by a group who had been organized for the purpose of recommending titles for purchase.
What we don't know (or at least, I couldn't find, so if you have a link to information, pass it on & I'll update accordingly): what, if any, existing collection development/ selection guidelines were in place; what is or is not in the current collection; the full list of recommended books and removed titles.
I won't get into the actual discussion at this point, except to say that the discussions have been interesting. Check out Here In the Bonny Glen's original post, and subsequent post (where I left comments that are way too long); Farm School; BookMoot, and I'm not sure which library she's talking about, but it shows what it feels like when it's the books you like that people don't want; and the Horn Book blog.
So, my two cents, such as it is: ideally, a school will have collection development policies in place and will follow them. See: School Library Collection Development; the ALA Workbook for Selection Policy Writing. One criteria: "favorable reviews found in standard selection sources." Traditionally, this has meant published journals.
Without further ado, here are some of the "standard selection sources" I use, in no particular order. They should be available at your public library or through a database that you can access via your library. Barnes & Noble, and to a lesser extent, Amazon, have paid the publishers for permission to use reviews on their website, so you can find them when you look for individual books. And, of course, they are available for purchase.
Oh, and it's not uncommon for the reviews to contain spoilers, since they are being used to assist in purchasing decisions. Some limited material is available on-line. Most of these journals also "star" exceptional material.
The Horn Book Magazine and the Horn Book Guide. Reviews children's and young adult; includes age level of audience.
Kirkus Reviews adult, children's, young adult. Reviews are known for being blunt -- if they don't like something, or think it's poorly written or plotted, they say so.
Booklist Published by the ALA; policy is to review only books they like (so if the book is not reviewed by them, it could be because it didn't pass muster).
School Library Journal Reviews preschool to high school; and professional materials. Their book review policy is here.
VOYA Voice of Youth Advocates Journal. Middle school & high school reviews. Reviews both quality (on a grade of 1 to 5) and popularity (on a grade of 1 to 5).
The Bulletin of the Center for Children's Books Review codes range from "special distinction" to "not recommended."
Some reviewers are paid; others are not. Many books are reviewed from galleys or ARCs. And there can be a wide range even amongst these; one publication may love and star a book, and another think it's a very narrow audience ("only for those libraries were science fiction is popular").
As for online reviews -- hey, you know who we are! But I'll give a plug to The Edge of the Forest.