'Counterpoint in Composition' began life not as a book, but as a collection of examples from the literature. Dr. Salzer and Dr. Schachter assembled it to show the connections between the simple procedures of species counterpoint and the complexities of composition, an approach to counterpoint deriving from the work of Heinrich Schenker.
Very comphrehensive volume offering a heavily Schenkerian approach to the voice leading issues... Includes for me very unusual techniques and views still based on the classical "Fuxian" counterpoint, but taking it in unexpected direction ("prolonged counterpoint"). If you don't have at least basic comprehension of Schenkerian thinking and methods, it's not book for you.
Best book out there for species counterpoint. The second half uncovers some very fundamental concepts in how harmony and counterpoint are intertwined and how the counterpoint learned in the first half serves as the backbone to functional harmony.