The RISC OS 5 Programmers Reference Manuals, commonly known as PRMs, are in the (slow) process of being made available online. Currently, only parts 7 and 8 (the Desktop? and Non-kernel input/output?), and FileSwitch are available. Part 15, is in the process of being uploaded. All other links from this page are likely to be non-existant. We’ll get there.
The RISC OS 3 PRMs in pdf form is freely available here.