Let me know if you find a wiki. I would love to have a place for RISC OS information but purely for the raspberry pi distribution(up to date PRMs, tutorials etc) but I don't think such a project exists yet
To a large extent that's true, there are "snippets" of information here and there. The PRM's distributed in the Pi RISC OS image go back to RISC OS 3.xx so are not quite up to date but do
give useful pointers and get across key concepts in RISC OS - but then miss out entirely on the new hardware abstraction layer and the 32 bit stuff.
A good (alternative) way to start is by looking at the "online" PRM wiki run by RISC OS Open - it isn't entirely complete but is
more up to date than the PDF PRMs in the Pi image. (Link to the online wiki is - https://www.riscosopen.org/wiki/documen ... %20Manuals
The RaspberryPi RISC OS distribution (RISC OS 5.19 RC8) is very similar to a normal RISC OS 5.xx distribution - the hardware is "abstracted" away by the HAL - and all the other stuff (largely) is like what you get on other RISC OS 5 computers - like the Iyonix. Now there are differences (Iyonix has PCI, a real-time clock/non-volatile RAM etc.,) - while the PI doesn't but for most common activities they're the same.
The online PRM wiki is probably the closest to the a "unified" PRM resource - but you'll still need to refer back to the paper/PDF version for some extended explanations or where parts are not yet completed in the online resource.
Hope that helps !