There is an incredible resource list in the back of each book. It is already organized according to the units of the book (not by age). The ISBNs are even listed to make ordering a breeze. We did the best we could to weed through all the books out there, but of course in science, many of the books are secular in nature and refer to evolution. Each family will have to decide for themselves how to deal with that issue. You will also find a cross in front of the books that are specifically Christian in nature.