I've been working my way through McLarty's book off and on for several months now. It is a tremendously clear and well-organized book, and you can learn a lot from it. HOWEVER: it is a "math book" in the strictest sense of the word. Exposition is kept to a bare minimum, and you have to actually work your way through the material (AND the exercises, since many of the definitions are given in them) in order to learn anything. He could have easier doubled or tripled the amount of exposition and still have produced a lean, mean textbook. This is a really good book if you need to learn category theory and you already know why. The only extensive example is a short chapter on group theory. After reading his article on category theory in the Routledge encyclopedia of philosophy I expected rather more in the way of theorizing. Be that as it may: everything you need to know about categories and toposes is in here, and nothing else. The best math book I've read in a long time.