If you do stat mech then this book is a must have. Yes, it's old but most of the problems are timeless as well as many of the topics covered. It is still a foundational book in my opinion.
I found this book to be extremely useful, though I do feel that reading the related chapters in McQuqarrie’s “Physical Chemistry: A Molecular Approach” was necessary. He goes quite in depth in the derivations and sometimes loses the conceptual aspect of the material, this is balanced by the P-chem book which, though targeted to undergraduates, is sophisticated enough to give a deeper understanding of the subjects treated in the Stat Mech book.
The classic Statistical Mechanics textbook. Definitely missing some derivations that would be nice (they are left as an exercise to the reader), but what can one do, right?