I look for biographies that make learning about famous people fun for kids. I especially like Childhood of Famous Americans, Opal Wheeler biographies, and Rookie Biographies. I know these are wholesome books.
I look for classic literature published with interesting illustrations. Because much of the classical literature is written in vocabulary that is oftenno longer used or difficult to understand, good illustrations help hold a child's attention. (I also keep a dictionary handy! Try reading Robinson Crusoe in it's original version and you'll see what I mean.)
I look for science and history books that make learning history fun - again with color illustrations if I can find them.
So much of what is in bookstores is full of violence, sorcery, vampires, sexual overtones, and bad language. I rarely find anything suitable in local bookstores. I do my shopping for children's books in Christian bookstores or used bookstores (both online and in local shops.)
I have a purpose for buying the book: Is it for my child to read on his own? Then I look for something that is below his or her normal reading level to make it fun and encourage reading. Is it to improve my child's reading skills? Then I look for something on or above reading level that we can read together. Do I want to inform or entertain my child? Then I buy a book that I will read to him or her. The vocabulary can be adult level because I can explain the words as we go along. This increases vocabulary.
I love books with pop-outs or inserts (like maps or diary entries) that you pull out and read. An example would be Lewis and Clark on the Trail of Discovery: An Interactive History with Removable Artifacts.
I hope that will get you started!