An Easy Course In Programming The HP-11C and HP-15C (1984) [pdf]
literature.hpcalc.org
literature.hpcalc.org
I bought a HP-15C soon after it came out in the early 1980s and used it for almost 40 years until it finally died. I felt like I had lost a piece of my identity --- but not any more. Thank you Bill Foote.
The idea was - find a great book, get the calculator that matches it, and proceed from there. I love learning offline with physical books and tactile things.
However, both the actual hardware HP-15C and the book are very exagerrated in price. Maybe instead I'll get some less popular model, there must bee others which aren't as well-known but that are similarly great.
https://www.hp.com/ctg/Manual/c03030589.pdf
At the time it came out (pre-smart phone), programmability in your shirt pocket was whiz-bang stuff. Obviously not so any more.
If you're seriously into programming, you'll probably be disappointed.
I'd recommend Forth instead. Forth is a RPN calculator on steroids.