My now 13yo ds went through an inexpensive sequence of iPad books - Trig., Precalc and Calc. published by a company called School Yourself (schoolyourself.org). I find their use of text combined with short animations very good, although I can't personally judge effectiveness as I already know the subjects well. I think they worked well for ds in explaining concepts, but they were very limited in the number of exercises for the reader. We have thus supplemented with "Calculus Workbook for Dummies," available for $11 from Amazon.