Ever wondered what is the cost of inheritance, virtual functions or exception handling? Is C faster than C++? How does the compiler know the type of an object at runtime?
This is definitely a classic. There are many other great books which are overlapping, in terms of content, with each other. But I don't know one of having information which it has.
The book itself can't be termed as dense as no code is without explanation/comments. Regardless, the material covered is quite advanced, so it will not be an easy read.
The book is quite old and does have some typos at some annoying places.
If you are a C++ programmer, or have an interest in C++, I think it is a must-read. Even if you are from Java background, and know a little C++, I think you would be a better programmer after reading it.