'musketjon' - It depends on your passion, interests, love of books, interest in history, and future orientation & direction in collecting.

IMHO:
A general collector of arms or a person, whose primary interest is in shooting and maintaining his rifle, may be happy with just the Poyer book.

Franklin Mallory's book, "The Krag Rifle Story", 2nd edition, has most of his Krag SRS Data, in addition to, Springfield Armory manufacture of the Krag (from Scientific American), evolution of the various Krag models, and good documentation of parts changes, finishes, and refurbishing.

William Brophy's book, "The Krag Rifle", is strong on the rifle trials that resulted in adoption of the Krag, good photographs, and very good coverage on Norwegian and Danish models.