I used this book as part of my second year course on analytical mechanics. I will summarise my thoughts on this book in a few bullet-points
*The book covers (roughly in order) variational calculus, virtual work, D'Alembert's principle, Lagrange's equations, the canonical equations & transformations, equations of Hamilton & Jacobi, relativistic mechanics and finally a historical survey.
*Lanczos is a remarkable writer, it is no wonder this book is considered a classic (albeit a lesser-known one.) A combination of an excellent writer and a fascinating topic.
*The style of the book is (somewhat) informal. He gives a more intuitive approach to most of the topics, including the motivation for each new concept, which is very enlightening (at least for myself.)
*Dover's catalogue describes this as a "philosophical" book. I would disagree with this - he dedicates perhaps three sections in total to the philosophical implications of the analytical method, and in one of those sections he states "[]...scientific treatises should avoid philosophical discussions, in the case of the variational principles, an exception to the rule may be tolerated...[]"
*This is not a book to use as a course textbook, more to "supplement" one's understanding.