I have just finished reading the book "Conceptual Mathematics: A First Introduction to Categories". It is one of the best mathematics books I've read. Not only because the exposed theroy is very interesting (indeed, I've learned deeper meaning of some things that I've taken for granted until now), but also for its exceptional presentation style: concepts are explained through many illustrated examples in an accessible way, without delving into deep abstractions.
If you want to read an accessible introduction to category theory, I can heartily recommended this book.
Tags: mathematics books