This is a book of fundamental importance. Though C.S. Peirce had early on realized that the Stoics had already anticipated what we would now call the sentential logic, it was Mates' breakthrough book that demonstrated that they had quite fully (though not systematically) grasped it. This is an important book, a work of impeccable scholarship (and in a field that is very difficult to work in).
His recent book on the Skeptics is no good, in my opinion. He was old when he did it.
The Leibnitz book is very good, but a bit eccentric.
(One last point -- for anyone who's interested: Peirce's name was pronounced "purse", not "pierce")