For a general and approachable, readable introduction to the long and involved history of the German language, Ruth H. Sanders' German: Biography of a Language is indeed both informative and enlightening, and does equally hold true to its title and what is promises, presenting the evolution of German as a language (and to an extent, to the Germanic languages as a whole, as an entity) like a kind of life story, through a series of essential, and interconnected important linguistic and historical turning points.
And of course, for the first presented and analysed turning point, Professor Sanders has naturally (and understandably) chosen the third millennium B.C., which is at present considered the approximate time frame for when speakers of PIE (of Proto-Indo-European) reached what is considered Europe (and actually, almost one eighth of German: Biography of a Language has passed before we as readers even encounter the first reputed and conjectured speakers of Proto-Germanic, on the coast of what is now Denmark).
Now I personally do find it majorly refreshing that the rather obvious fact that we still do not even remotely know with any certainty what exactly caused the Germanic languages to branch off from its sister languages, from the main family tree of Indo-European, that this is readily accepted and clearly delineated by the author (that we do not really know what specific linguistic our historical phenomena for example caused the so-called first or Germanic sound shift, what caused the Germanic verb endings for the regular or "weak" past verb tenses, and from which language or languages the almost one third non Indo-European but pan Germanic vocabulary dealing with such themes as the ocean and seafaring culture came or perhaps more accurately, were borrowed, loaned and gleaned).
And Professor Sanders does point out in German: Biography of a Language some of the very many theories that have arisen since the 18th century to try to explain the above, to specifically consider the differences between the Germanic languages and its Indo-European sister languages, but Sanders always and fortunately specifies that these are all (and even the most recent attempts at that) mere conjectures and that no proverbial smoking gun with regard to how the Germanic languages became what they are, and specifically, how the differences between them and say Greek, Latin, Sanskrit, Celtic and the like came to materialise, has as yet been found, although she does demonstrate that for the most part, languages do not tend to spread because newcomers, invaders, settlers simply replace indigenous populations, but rather and more likely that the latter end up adopting the newcomers' tongues and dialects, often though adding their own languages' influence, in particular with regard to pronunciation, as well as with vocabulary choices (especially with regard to objects, with regard to phenomena that are new for the settlers and for which their own language thus would likely not have the required descriptive and prescriptive words).
And to continue, I do have to admit that on an entirely personal level, I have found the second of Ruth Sanders' described turning points in German: Biography of a Language more than a bit problematic from a cultural and political standpoint. Yes, the defeat of the Roman legions at the Battle of Teutoburg Forest in 9 A.D. (by Arminius aka Hermann) did certainly prevent, did in fact stop Roman expansion eastwards and northwards across the Rhine. And it is indeed both the truth and interesting to realise that in the romanised areas of Central and Southern Germany, while the population readily seems to have adopted and even embraced Roman cultural, political practices and lifestyles, Latin as a language did not simply replace the extant Germanic dialects (unlike in what is now Spain and France, where aside from some place and river names, Celtic was basically across most of the conquered areas totally replaced with and by Latin, in the long run giving birth to the Romance languages, to Spanish, French etc.). However, even if Arminius and the Battle of Teutoburg Forest was as such likely an important linguistic and historical turning point (event), considering the rather overt and hero-worshipping Arminius and Germania cults that became so prevalent in the Third Reich, I have found this particular section of Germany: Biography of a Language while interesting, also more than a bit personally uncomfortable.
And while of course, the split of the Western Germanic languages, the Western Germanic dialects into high (alpine, southern) and low (northern, coastal) varieties is another important and of much linguistic interest turning point (especially with regard to the second or old high German sound shift, which is the main point of separation between high and low German, including for that matter Anglo Saxon), for the German language as a whole, for today's standard German, the most celebrated and yes the most essential (and as such also covered the most extensively and intensively in German: Biography of a Language) is without a doubt the literary work and influence of Martin Luther and his Bible translations (from Latin into German vernacular). Luther's linguistic importance and influence has of course almost since the beginning of historical linguistics been both noticed and readily accepted, but Ruth Sanders makes an important point stating that Luther did not simply translate the Bible into some randomly chosen German dialect (East Middle German), but that he specifically combined "Kanzleideutsch" (chancellery German) with East Middle German, a dialect that in many ways truly straddles the middle and thus contains aspects of both high and low German dialect peculiarities (so as to produce, so as to provide to the potential readers of his translated Bible a German vernacular that could be learned, that could be understood by most or rather by the most individuals, their different and varying dialects notwithstanding). And since many Germans especially from the 15th to the late 19th century often actually learned to both read and write from Martin Luther's Bible translations, his combination of chancellery German and East Middle German thus also and relatively quickly became the basis for what we now label as modern standard written German (and in ALL areas of Germany, mind you, and not just in those areas traditionally deemed to be Protestant). Furthermore, Luther's written German has also lastingly affected the spoken tongue, the pronunciation of German, and so much so that in many ways, Germany's dialects are today not nearly as robust, not nearly as vibrant as erstwhile, simply because with much of the population learning how to read and write from Luther's Bible, the spoken word, the pronunciation of German has also become more and more standardised, even within the dialects themselves (and although it is true that recently, there has been a concerted effort to protect and revitalise Germany's many dialects, some of the "damage" has likely been permanent, with Luther's standard becoming both the written and the oral comme il faut).
But annoyingly, I do have to say that the final two sections of German: Biography of a Language (basically the post Martin Luther chapters) do feel rather rushed and are in my humble opinion not covered by Ruth Sanders nearly as extensively and as intensively as the history (or should I say the biography) of the German language up to and including Martin Luther. And while I suppose that there is enough basic information presented, when one then considers how meticulously and specifically detailed Professor Sanders' presentation of Martin Luther's influence on the development of a standard written German has for example been, it is more than a bit disappointing that for example the role of German as a language weapon during the Nazi era is described in something like two odd pages (I do know and yes, even realise that how the Nazis approached the German language, how they abused and manipulated it is a potentially explosive and in all ways majorly uncomfortable topic, but it is nevertheless a linguistic occurrence, a historical situation that did happen and as such remains a scenario that has also influenced the development and yes the international reputation of modern, of contemporary German and in my opinion Ruth Sanders should really have devoted more than simply two pages to this).
And finally, another personal albeit minor disappointment with German: Biography of a Language is that alongside the author's tendency to repetition, to at times presenting already covered information in a new habit, in a new costume, the accompanying timelines and tables most definitely contain far too many errors and typos and could really have befitted from some editing. However and all that being said, I do and very much so still consider German History of a Language a both enjoyable and delightfully academic (but thankfully never too textually, philosophically dense) read, and as such worthy of a high three star rating (and while some knowledge of German and especially of German grammar rules etc. are perhaps an asset, one can in my opinion still enjoy and learn much from Ruth Sanders' narrative, her text, even if one is unilingually English speaking/reading). Recommended to and for those readers interested in historical linguistics and the history of German and the Germans (with the caveat NOT to consider the Kindle edition of German: Biography of a Language, as the transfer from paper book to e-book format has been less than stellar, with especially many of the timelines and tables rendered almost unreadable and useless).