The "Burns & Welling" book covers concurrency in depth, but doesn't have as much to say about distributed systems as I would expect. Nevertheless it is probably essential reading if you're going to be doing a lot of this stuff.
I'm still reading Professor McCormick's book "Building Parallel, Real-time and embedded Applications with Ada" and it does an excellent job of getting a reader started with a wide range of application-oriented aspects of Ada - sadly missing in other books which focus o the base language - and that includes both the DSA (pure Ada) and PolyOrb (for mixed languages) approaches to distributed systems, including very readable code examples.
Start with this latter book (IMO). (and its lead author has been seen around these parts, so this is a good place to ask questions! :-)