This is the sequel to Mann's translation of part one of Fux's "Gradus ad Parnassus", (which is the section on species counterpoint, and which is a prerequisite to this book.) "The Study of Fugue" includes the best of Fux's instruction on fugue (part two of the "Gradus ad Parnassus"), plus additional material from 3 more important 18th century writers, two being the teachers of Mozart and Beethoven respectively, and one an associate of J. S. Bach.
Alfred Mann's detailed history of fugal theory makes up the first part of the book. If you combine this with the similar exposition in the first part of Knud Jeppesen's "Counterpoint", you have a wonderful overview of the history of contrapuntal music theory from the medieval period up to the dawn of romanticism.
Jeppesen's "Counterpoint" does contain a short appendix on fugue, but many details are left out. Although Jeppesen's book is a valued resource in my library, reading Mann on fugue exposes two shortcomings in Jeppesen which are not apparent to the reader of Jeppesen alone (I reviewed Jeppesen's book separately but some comments are relevant here.)
(1) Jeppesen's insistence on correcting Fux's faulty understanding of Palestrina's style (which he does in exhaustive detail) misses the point that the species exercises are only preliminaries to writing Baroque fugue, and are not really intended to reproduce Renaissance style - that is, Fux's use of Palestrina as a model is more a literary device than a practical exercise. So you can give Fux a break; Jeppesen is right about the details of Palestrina's style, but Fux's book is still valid in itself as a manual of Baroque practice.
(2) Jeppesen fails to properly or fully explain the function of the plagal and authentic modes as the organizing force behind the ranges of the four voices - soprano, alto, tenor, bass - and the huge effect that this organization had on the evolution of the fugue by in turn organizing the development of the "tonal answer". One finishes Jeppesen's book still wondering why the plagal modes existed at all; one finishes Mann's book understanding that the "twelve modes" of Glareanus (six plagal and six authentic, an expanded set of the original eight medieval "church modes" which was current in the 16th and 17th centuries) were an active part of musicians' thinking well into the 18th century, and understanding that this in turn is important to understanding fugal practice.
Each of the four 18th century authors showcased by Mann has his own repertory of fugal tricks and techniques, and they fill in each other's weak points pretty well, so it's a blessing to have them all together in one book. The fluidity of fugal practice is emphasized, and it is clear that there are lots of ways to write a fugue, and that there is no one absolute formula which must be dogmatically adhered to.