Has too many errors, misinformation, and non sequiturs to be trustworthy -- the author, by his own admission, has no background in language-study. Sections on Proto-Indo-European are preceded by paragraphs devoted to dinosaurs (!), the Ice Age, and primitive man; family trees of the Indo-European languages show Welsh descending from Irish, Armenian from Persian, and Old English from Old Saxon; Gothic is considered a dialect of Old High German, whereas Icelandic is listed under "related dialects" of Low German; Gothic and Old Norse and other linguistic forms are given conjugated or declined forms, but not cited as such; the terms "German" and "Germanic" are used interchangeably (the former means "Deutsch," of or belong to the country of Germany, the latter means "Teutonic"). I am not myself well enough informed to speak on the degree of accuracy concerning the main topic of the book, Plattdeutsch (Low German); but the many errors in areas in which I AM informed make me unable to trust the author on things which I am not informed.
In an introductory note the author makes the observation that scholarly works often pass by a potentially-interested lay audience by being too technical, written only for scholars; the complaint and the problem are legitimate, but surely the solution is lay-books written by scholars, not lay-books written by other laymen? Non-scholarly audiences have as much a right to correct information as scholarly ones.