Resources (books, etc) to learn about concurrency theory I want to know more about concurrency theory from a formal/mathematical point of view, I know there are a lot of computer science branches that relates to concurrency theory like process algebra, Petri nets, Actor model, CSP, etc.

But I don't know about classic books on that topics, and also I would like to know in which order it's better to study, I'm new to this field, I have some basic knowledge on λ-calculus and Turing machines theory. 

What resources do you recommend to begin with this 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.