The best book on programming (SIBO and EPOC) Psion machines is "Programming Psion Computers" by Leigh Edwards. It covers wide range of topics, including SIBO architecture, available programming tools and detailed OPL reference. And it's free
You can download it from Gareth Saunders' website, as well as the OPL reference in various formats (including Psion's database file).
For learning OPL you can also take a look at the tutorials devoted to programming Symbian machines (this one, for instance).