Basic Category Theory is an introductory category theory
Publication and purchase
The book is published by Cambridge University Press. You can find all the publication data, and buy it, at the book's CUP web page. It was published on 24 July 2014 in hardback and Kindle formats.
By arrangement with CUP, a free online version is now available as arXiv:1612.09375.
The arXiv version is not only free; it is also freely editable. So, for instance, you can alter it to suit the needs of a course you are teaching or a device you are reading on. This page of the arXiv version tells you more. It is released under the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International licence (CC NC-BY-SA 4.0).