Not a recipe book of calculations, but a systematic exposition of the principles of statistical mechanics. Unparalled in its ponderousness, the derivations are beautiful, and the physical motivation for each one is always laid out in detail. This covers quantum as well as classical statistical mechanics. In fact, even the part on quantum mechanics alone is worth reading, because it gives insight into the state of quantum mechanics in the 1930s, as viewed by one of the deepest thinking physicists around. Back then, the fundamentals were plainly in sight, whereas now physicists are lost in the mathematical thicket.
Naturally, since it was written so long ago, it is not by any stretch the final word on statistical mechanics. It's still a masterpiece, which illuminates many deep questions in physics. This is not, repeat, NOT, just a book of historical interest.