Reading material
Lecture notes on CCS.
Additional material
More about CCS can be found in

Robin Milner.
A Calculus of Communicating Systems,
volume 92 of Lecture Notes in Computer Science.
SpringerVerlag, Berlin, 1980.

Robin Milner.
Communication and Concurrency.
PrenticeHall, Englewood Cliffs, NJ, 1989.

Robin Milner.
Communicating and Mobile Systems: the
calculus.
Cambridge Univeristy Press, 1999.
Question
A labelled transition system consists of

a set Q of states,

a set A of actions and

a set of transitions. Each transition consists of a start state, an action
and an end state.
Labelled transition systems are very similar to finite automata.
The main differences are that the set of states and the set of actions
of a labelled transition system need not be finite, and that a labelled
transition system does not have a designated start state and a
set of final states.
When should two states of a labelled transition system be considered
behaviourally equivalent?