What book will suit you depends on your background. If you have a solid background in math, say an undergraduate degree in math, my favorite book is Emily Riehl

What book will suit you depends on your background. If you have a solid background in math, say an undergraduate degree in math, my favorite book is Emily Riehl's Category Theory in Context [1] (the "context" in the title refers to a background in math). If you don't know that much math, I'd recommend Tom Leinster's Basic Category Theory [2] or Steve Awodey's Category Theory [3] (particularly if you have some background in computer science or programming). [1] https://math.jhu.edu/~eriehl/context.pdf https://math.jhu.edu/~eriehl/context.pdf [2] https://www.maths.ed.ac.uk/~tl/bct/ https://www.maths.ed.ac.uk/~tl/bct/