Any experienced users of hp calculators in Berlin?

[rprosperi]

I am not in Berlin (though I very much wish to visit someday) but I suggest you try this book, written by Bill Wickes, the overall project manager and architect for the 28S Operating System, RPL.

https://literature.hpcalc.org/items/1590

It is more of a tutorial format than a manual's approach, and his writing style is comfortable and easy to follow along, building upon each step. He also wrote similar books for the HP-48SX and -48GX which are IMHO even better, but those models are sufficiently different that these may not be helpful. But if you like RPL, I strongly recommend you explore getting one of these models as RPL matured a lot when growing from 28 - 48, where it became more flexible and powerful.