It is amazing the vitality in our admittedly small and "vintage" hobby. As soon as a new edition of a vintage calculator appeared, there have been people working on it.
I plan to give some of these HP15c calculators to my children (I will buy a decent stock of them for myslelf), but it is good that there is already a programming tutorial for it. I have included links to it in my Manuals and Support page. Thanks, Eddie for it!
HP15c Programming Tutorials from Eddie´s Math and Calculator Blog
Chapter 1: The Stack
Chapter 2: Basic Programming
Chapter 7: Solve and Integrate
Chapter 8: The summation program
Chapter 12: Memory and indirect registers
Chapter 13: Indirect Addressing and subroutines