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.
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
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.