The age and mathematical rigor (not to mention the fact that it is translated) all add to the difficulty of this text. However, with some effort, one can gain a great deal of insight into the mathematics behind statistical mechanics. The book is filled with proofs and theorems and is a great resource for mathematicians. For a physicist, however important these foundations are, the connexions between the proofs and the end result, the applications to the real world, are a bit labored throughout most of the book. Still probably worth having on your bookshelf.
This book is quite challenging, but mostly because I don't know a lot about mathematics. If I knew more about grad and Lebesgue I suppose it would come out better. As I stand, I can't understand this book all that well, and I need to have a stepping stone for it. Sometimes being an autodidact is tough. No classes or anything really makes things complicated.
On the other hand, the book is written pretty well, it's mostly equations and text. Not a lot of pictures, it somewhat explains the units and symbols used, but I just don't have the background for it. Maybe if I revisit some Calculus and Linear Algebra and then come back to this book again.