A number of pages provided by Oracle - some will require registration, Metalink requires a support contract.
Howard J. Rogers has written some excellent documents on Oracle, in particular I'd recommend his "Backup & Recovery" guide and his "Oracle 9i New Features" guide. Unfortunately he's had to remove them from the web. I can't link directly to them, but you may be able to find them by searching with Google - it may also help to know that HJR usually distributed his documents in PDF format.