Even if John T. Waterman's A History of the German Language is indeed more than a bit dated in places, and as such could and perhaps even should be considered for a bit of an update (seeing that it was first published in 1966, the second and revised edition in 1975 and both editions thus of course prior to the 1993 reunification of West and East Germany as well as the orthographical reforms of 2000, the Neue Deutsche Rechtschreibung), this handy study and research tome is still and in my humble opinion amongst the most easily and painlessly approachable monographs of German language history available at present. And as such, A History of the German Language is both academically and from a general reading pleasure and value point of view very much a beloved and appreciated personal favourite, a book that unlike some of the more popular recent language "biographies" does manage to retain a primarily academic focus throughout (especially with regard to language structure and sound shifts, how in particular grammatical endings, how verb conjugations and noun/adjective declensions have changed, have altered since Proto-Indo-European times) whilst at the same time also featuring both a delightfully and appreciatively smooth and easy to peruse, easy to understand structure and general presentation (and one that makes in particular the two main consonant sound shifts, the Germanic and the High German shifts appear both logical and clearly delineated and explained, which from my own reading experiences has most definitely not always been the case with both recent and not so recent examples of German language history books and unfortunately with both German and English language offerings at that).
When I was preparing for my First Comprehensive Exam for my PhD at the University of Waterloo (this happened in 1996, and the First Comprehensive Exam featured both standard linguistics and specifically German language history, as well as German literature from the Mediaeval era to the end of the 15th century), I actually read and consulted a goodly number of library books on linguistics and German language history during the many weeks of heavy-duty and intensive preparation. However, it was really only Waterman's A History of the German Language that I found lastingly beneficial and to such an extent that I even purchased a copy for my personal library (also because I was getting pretty sick and tired of the fact that the library book I had signed out kept getting recalled over and over again). And it was indeed this very tome, it was indeed A History of the German Language which I not only (and obviously) consulted the most, it was also the one main research monographs of the many I had checked out of the library and read that provided me with the essential necessary information and enough of a basic linguistic and language history understanding of German to pass my First Comprehensive Examination with very much and surprising flying colours so to speak.
And therefore, even considering the minor issues with regard to datedness (which is also and by simple necessity and reality the accepted nature of the academic beast), I still and very highly recommend Waterman, and absolutely do consider his A History of the German Language a simply and utterly perfect basic level research and study guide for especially undergraduate and graduate college and university students of German, and as such rated and ranked with five most well deserved stars, both for its presented, featured text and also for its intensive and extensive bibliographical lists (with the necessary caveat that while indeed an amazing and extensive treasure trove of academic works on the German language and its history and historical significances, the bibliographical listings do, of course, cease post 1975 in this here, in the revised edition, and thus, the many linguistic and language history books published since then, will of course and by simple deduction and reasoning, not be included).