Imagine discovering Aristotle's lecture notes on a class he took from Plato. This book is very nearly as momentus, and no doubt scholars 2,000 years from now will be studying with as much interest as we're reading it today.
The introductory material is quite enlightening: not only does it explain Frege's bizzaro 2D notation for his system of logic, it also gives many anecdotes about what Frege was like as a person and what it would have been like to be a student in his class.
Frege is credited with the first real substansive advance in logic since Aristotle, and Carnap was really kicked into a higher orbit after taking this class, going on to become a towering figure in 20th Century philosophy. This book provides fascinating view into one of the most important logic classes which took place last century.