This textbook is hardly to be surpassed by any other treatment of the subject. Originally written in 1937-1939, the treatment provides a bridge between the classics (Maxwell, Boltzmann, Gibbs) and the then new world of quantum mechanics.
What really impresses me is its modernity in approaching the subject from the full quantum point of view, using quantum statistics and density matrices from the outset. This is unifying and economic at the same time. Moreover, time and again I have found that many "modern" concerns in equilibrium and nonequilibrium statistical mechanics, like, for instance, the issue about the equilibration of quantum many-particle systems, echoes opinions and insights already stated in this textbook. Everytime I pick my copy to take a glance I realize how puny such acclaimed books like L. Reichl's or the new M. Kardar's ones are (despite their many merits).
There may be more gentle introductions to statistical mechanics out there (choose one), but this book makes an ideal 2nd. reading. This is the book that, together with S.-K. Ma's and K. Huang's books on the same subject, will make you a pro.