To understand SOA in general I would recomment the ones by Thomas Erl and Paul C. Brown.
When starting with the Oracle SOA Suite:
* "Oracle SOA Suite 11g Handbook" by Lucas Jellema eg via Amazon
I started with The Definitive Guide to SOA: Oracle Service Bus (Apress). It's very hands-on, lots of code samples that allow you to get a handle on things. If you want a real theoretical background maybe try some of the other books suggested, but I found it wasn't necessary, this was enough to get me going.