This book provides very good foundations over almost all of the whole area of filter design. It should be stressed that this book is not a filter cookbook for a casual design - it is a very serious guide to the mathematics underlying filter design, starting with a discussion of the Laplace transform, and moving to passive filter design and analysis.
Budak enables the reader to acquire a very aquaintance of the limitations involved with network synthesis techniques - exhaustive discussion of issues such as sensitivity (tolerance to component inaccuracy) appear, with a section devoted to component non-ideality.
It should be noted that the book was originally published in the early seventies; perhaps the only thing missing from the text is a more contemporary discussion of design techniques applying to active high frequency circuits.
Twenty five years ago, video op-amps were not particularly thick on the ground, and this situation has changed a good deal for the better.
Another thing which would be very welcome would be a discussion in equal depth regarding the analysis and synthesis of circuits which consist of a mixture of distributed and lumped components.
All in all, a very complete and satisfying treatment of a broad subject.