PRISM appears to be more relevant to magazines than books. It may be useful in encouraging a move to epub instead of relying on custom-built apps, though.
I don't really see the point of ONIX, but then I'm not an accountant. ONIX has certainly added a few new epub-specific tags and looks like it's aiming to get integrated. But a lot of the information it covers it potentially quite sensitive - publishers and distributors aren't going to be entering discount codes and the phone no.s or email addresses of employees into an epub to be read by anyone. ONIX looks like a schema for a back-end stock-management database, I don't see why it should be part of the epub spec.