As someone who, like Max Reger, "lives inside fugues", I can't recommend this book.
The jacket description is accurate. This book is a collection of essays describing in both subjective and objective terms 16 of JSB's fugues. What I mean by this is -- do not be misled by the (in my opinion, overreaching) title, that this is a general work describing abstract fugues and their artistic value. It's a collection of "reviews". The disjoint nature of the essays is not in itself a fault, as this is not marketed as a novel. The author's voice constantly straddles the line between technical and popular, which as a non-professional musician I thought would appeal, but instead feels tedious to read through. This, combined with the essay format, create an odd reading experience, somewhere between erudite academia (say, published in the BACH journal) and casual, recreational reading (like Gardiner's: "Bach: Music in the Castle of Heaven").
In the preface, the author states "readers will need the sheet music with the bars numbered". Unless the reader is intimately familiar with all the pieces, this is an absolute prerequisite, given the relative dearth of inline music examples. The scores are given on the CD and are obviously available on the internet, but Kerman could very well have made his most important points with more excerpts throughout the text. I find this problematic, as similar general audience texts (Poli's "The Secret Life of Musical Notation") are much more effective as single-volume, self-contained books that don't require flipping through other texts. (Complicating the matter not trivially is that Kerman's selection comes from four (five?) different major collections--WTC I & II, English Suites, Kunst der Fuge, misc works--meaning that even as someone who owns it all in separate "complete" editions, I had to drag from my piano five different volumes...)
Reading through the essays gives a certain insight to the composition and certainly guides you in what to look for in other pieces. Kerman does a good job at cultivating an appreciation for the technical minutiae and emotive appeal of the fugue to those otherwise underexposed to it. He is unafraid of criticizing pedestrian moments in Bach's music (calling parts of WTCII B-flat major "decidedly anomalous" and "hard to digest"), which is a very positive trait, as putting anyone--long-dead composers included--on a pedestal is harmful and undermines other, genuine assessments.
Overall, it is hard to say that this book really fulfills any need in the literature on either Bach or fugues in general. I suppose it is a nice, casual volume to introduce someone, say, a non-musician who just got into Bach's music, or who enjoys listening to string ensemble arrangements of the Goldbergs, to the aspects of fugues worth investigating and listening for.
For Bach's keyboard works, I would turn to Schulenberg's consistent, if not somewhat clinical, treatment and description of all of Bach's non-organ keyboard music in "The Keyboard Music of J.S. Bach". For an exegesis on fugue in general, well, I would agree with Kerman that the so-called popular literature is lacking outside of austere counterpoint textbooks, but unfortunately his book does not fill that hole. Or, perhaps that is an artificial hole, and fugue is itself a technical notion that must be approached with technical description if we are to say anything more useful about them.