Resources (books, etc) to learn about concurrency theory
There are not that many books on this subject, as it continues to evolve at a rapid pace.

Classic books on process calculi (that don't focus on &pi;-calculus-like mobility) are:


C. A. R. Hoare, Communicating Sequential Processes.
A. W. Roscoe, The Theory and Practice of Concurrency.
M. C. B. Hennessy, Algebraic Theory of Processes.
R. Milner, Concurrency and Communication.


There are two main books on &pi;-calculus:


R. Milner, Communicating and Mobile Systems: The Pi Calculus.
D. Sangiorgi, D. Walker, The Pi-Calculus: A Theory of Mobile Processes.


The Sangiorgi/Walker book is not easy reading. The others books above can be used by a learner.  

All of the books above are (mostly) about untyped processes. There is a single book on types for interactive processes (from June 2017).


S. Gay, A. Ravara, Behavioural Types: from Theory to Tools.


Regarding recommendations, that would depend on your background and specific goals. The Milner and Hoare books should be easy to access. After those you can read the original papers.