From the reviews: "The main and basic point is that category theory is thoroughly geometric. ... The monograph is highly readable, and it should be valuable to every expert in category theory, every novice in category theory and every mathematician who is not interested at all in category theory." (Hirokazu Nishimura, Zentralblatt MATH, Vol. 1165, 2009) "The creation of a book such as this marks an interesting point in the history and philosophy of category theory. ... While reading the book, I used the opportunity to revisit several seminal papers and to marvel at the advances they represented. The author clearly studied each paper thoroughly, had private communications with some pioneers, and succeeded quite well in describing the mathematical community's mind set at the time of those advances. ... I conclude by recommending the book." (R. H. Street, Mathematical Reviews, Issue 2012 d)