HP-28s

[rprosperi]

Bill Wickes' excellent book "HP-28 Insights Principles and Programming of the HP-28C/S" is almost certainly the best book, however it's fairly rare and hard to find. But, some good news is this book is included in the MoHPC USB Drive collection, available here.

This collection also includes the analogous books for the 48SX and 48GX, which are extremely similar (though not exactly) and they provide additional insights which can be used when working on the 28 machines.

"HP-28S Software Power Tools: Utilities" is also included though I've never read it so can't comment on it specifically.

And of course it also includes all the HP manuals and Solution Books too.

This collection is probably the best value you'll find for resources like this.