What is best resource and Where to study countability topic in toc?

Countable Set is a set having cardinality same as that of some subset of N the set of natural numbers . A countable set is the one which is listable.

In mathematics, a countable set is a set with the same cardinality (number of elements) as some subset of the set of natural numbers. A countable set is either a finite set or a countably infinite set...

Cardinality of a countable set can be a finite number...

For example,

B: {1, 5, 4}, |B| = 3, in this case its termed countably finite or the cardinality of countable set can be infinite.

For example,

A: {2, 4, 6, 8 …}, in this case its termed countably infinite...

https://cs.uwaterloo.ca/~watrous/ToC-notes/ToC-notes.02.pdf