I just found out that the author of Calculus by and for Young People died this year:
Donald “The Mathman” Cohen was a great educator and influenced thousands of children and adults throughout his life. He began teaching in the 1950s and continued his work through The Math Program which started in 1976 in Champaign/Urbana, IL with partner Jerry Glenn. Don continued teaching right up until he passed away in 2015 at age 85. Don’s books and other materials continue to be treasures for all who are interested in math or math education.[Don Cohen – “The Mathman” – 1930-2015]
His family is making his books available as free PDF downloads, as a memorial: Don Cohen – “The Mathman” – 1930-2015
We didn’t use his books (by the time we found out about them, our son had already acquired enough calculus not to benefit from the books), but lots of kids did get an early start on calculus using them. He aimed is material at middle school students, rather than college students. His approach to teaching math seems to have been highly regarded both by math teachers and in home-school circles. More info about his math program at his web page.