"The Systems Bible", also known as Systemantics, is a classic and highly recommended. It's probably not what you expect: it covers systems in general, and large complex systems in particular. It's not specific to computer systems let alone any particular flavor of computer system - these are rules which apply to all systems, from a municipal garbage collection program to a supertanker. Yet if you've ever dealt with a real-world distributed system you'll find it surprisingly relevant and timeless.
I suspect Level 4 is to read Distributed Algorithms by Nancy Lynch, who won the Knuth Prize for this stuff. A shorter book that looks good is Distributed Systems: An Algorithmic Approach, by Sukumar Ghosh. I recently bought them both but haven't tackled them yet.
"Enterprise Integration Patterns: Designing, Building, and Deploying Messaging Solutions" is a classic for messaging. It's pretty advanced I guess, but beginners can understand it too.
It is also free.
After learning the syntax, check out these chapters
* "Designing a Concurrent Application" http://learnyousomeerlang.com/designing-a-concurrent-applica...
* "Buckets of Sockets" http://learnyousomeerlang.com/buckets-of-sockets
* "Distribunomicon" http://learnyousomeerlang.com/distribunomicon
* "Distributed OTP Applications" http://learnyousomeerlang.com/distributed-otp-applications