In the introduction of Jean-Luc Brylinski's book "Loop Spaces, Characteristic Classes and Geometric Quantization it says that to read this book one should have a basic knowledge of point-set topology, manifolds, differential geometry, graduate algebra, be familiar with basic facts regarding Lie groups, Hilbert spaces and **categories**.

So what is a good resource to have a first look at category theory in this context.

The book describes among other things two levels for looking at degree-$3$ cohomology theory, and the more abstract one involves sheaves and groupoids which is where I suspect category theory kicks in (?).

In particular I would be also thankful if somebody could tell me what I need to look at first in the context of the book...