Before going to a library to borrow some books for my 10 year old son and 4 year old daughter, I usually search around and determine a list of books I would like to borrow.
There following is a list of tools that I found very useful.
2. shelfari
3. Goodreads
4. Amazon
With above tools, I can easily find books that fit the age and have better reviews and then check whether I can get it from my local library.
There are some other good resources to help parents to find appropriate books for kids as well, such as
1. http://www.teachingbooks.net/
So far, the best series of books both my kids enjoyed are the the Berenstain Bears. You can find more information from their official web siste http://www.berenstainbears.com/. Most libraries have them, I am pretty sure.