We have the first three books on order for our DD9.5. I, too, was unsure which books to get. On the website, the author suggests that if multiplications are not memorized to start at the beginning. Sorry, I can't offer any more help... we're still waiting for them to arrive. I figured that if they are too easy for DD, that DS3.5 will be able to use them soon, so no money would be wasted either way.

Tomorrow is always fresh, with no mistakes in it. — L.M. Montgomery