This is the most authoritive 19th century history of the development of modern probability theory. Modern readers will not find it easy to read. By modern standards, the prose is dense. The mathematical notation is somewhat antiquated. But for those who are interested in where modern probability came from, it is indispensible. The book was reprinted a few years ago in a bulky paperback edition. There are also electronic, PDF, editions on the web. Todhunter was a very interesting chronicler of the development of mathematical ideas. He also wrote on the history of the Calculus.