When I glanced over the upper ones, I did see one or two places in the book that contained a word or two that I wished were not there(we are on the cautious side about book content), though overall they were fine. Guess the author probably had middle school and up aged kids in mind when he wrote those relatively higher level books.
Mr. Schmidt thinks algebra is not for pre-pubescent children: "There is an old saying that you shouldn't start algebra until you have hair under your arms. A child's brain needs to develop physiologically before tackling the abstractions that algebra contains."
http://www.stanleyschmidt.com/FredGauss/32%20pre-algebra.html . I disagree with him and buy his books for my 7yo boy anyway.
