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
forget Thomas Erl, you will find yourself reading ten times the same paragraph asking yourself "what the heck is he trying to say???" and then finally you will fall asleep.
I like Gregor Hohpe "Patterns Of Enterprise Integration", his site http://www.eaipatterns.com/ is really cool and full of very practical patterns and icons.
There are a few nice books here http://www.javamonamour.org/search/label/books
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.