Notes on Category Theory by Robert L. Knighten.
From the preface:
There are many fine articles, notes, and books on category theory, so what is the excuse for publishing yet another tome on the subject? My initial excuse was altruistic, a student asked for help in learning the subject and none of the available sources was quite appropriate. But ultimately I recognized the personal and selfi sh desire to produce my own exposition of the subject. Despite that I have some hope that other students of the subject will find these notes useful.
The other generous explanation for “another tome” is to completely master a subject.
Either teach it or write a master tome on it.
I first saw this in a tweet by OGE Search.