In the www.hpmuseum.org site, which is probably the website most popular when it comes to HP calculators, Han has published a couple of very interesting tutorials for Prim programming.
You can find the first here: HP Prime Programming - An introduction
It shows the answer to many questions I have faced when starting to program. Questions that imply that Han has a deep knowledge, not only about the manual, but about the behaviour of the machine. There are many things that do not appear in the manual but are the result of testing and failing or succeeding with the calculator.
A comment that can be made is that most of the explorations have been made with the calculator emulator on the computer. There are many things that can be done quite easily using your fingers on the calculator screen, like bringing variables to screen, executing programs, etc. For example, when he speaks of the several methods or running a program, he does not cite the one I use most: locate the program with the fingers on the toolbox menus! That said, I have learned a lot of things about the Prime when reading it, and it is for me probably the best introduction to programming the HP Prime.
The new software used for the forum in hpmuseum.org allows to easily include code snippets, that do not occupy substantial space but scroll in the mini-window allocated for them. This is quite convenient to copy programs to illustrate a point, without interfering with the reasoning behind. This is used by Han to very good effect.
There is already a continuation in HP Prime Programming: Variable types and their priorities