In all my years of working in Ingres I have never found a book on the topic, never mind a good one!
The online documentation that comes with Ingres is very good. have a read og the getting started and Database Administrator on-line pdfs and they should give you a good idea.
Generally there are quite a few fundamental differences as to how the data is stored and terminology used, but if you know how to use oracle, it should not take that long to adapt.
Good luck!