Salmons proceeds chronologically, with chapters on prehistory and Indo-European, Germanic, Old High German, Middle High German, Early New High German, and New High German. He does, however, highlight the problems of this periodization and its artificiality, discussing for example the creation of a standard version of Middle High German by Karl Lachmann in the early 19th century. There's a big emphasis throughout on variation, regional and social as well as temporal, and on the unevenness of change. And he stresses the limitations of our sources for the earlier periods ("the entire surviving OHG corpus consists of about 330,000 written words"), especially for understanding temporal change and regional variation. There is no attempt to be comprehensive, with a few topics in phonology, morphology, syntax, vocabulary, dialect variation, and sociolinguistics chosen for attention in each period.
Salmons touches on the background of migrations and on the earliest texts, but the chapters on Indo-European and Germanic necessarily centre on phonetics and phonology. He gives a rapid introduction to historical linguistics and the reconstruction of languages, then considers topics such as the Germanic "accent shift", consonant and vowel changes from IE to Germanic, Grimm's Law and Verner's Law. He also looks at the basic structure of IE nominal classes and their Germanic forms, at verb groups and ablaut (the stem vowel alternation visible in strong verbs).
"Another view of the shift, one that some may find a useful mnemonic in learning the connectedness of the steps of Grimm's Law, treats the shift as having links connected by pushing rather than pulling, with each element moving one step, illustrated by the labials:
*bh > *b > *p > *f
This view is perhaps best motivated by the 'markedness' of the voiced aspirates. Some linguists argue that sounds that are complex phonetically (requiring a configuration of the vocal folds that many languages don't have) and phonologically (marked by two features, voice and aspiration) are more readily lost over time. On this view, in an effort to rid themselves of *bh, *dh, and the rest of the series, speakers began to simplify them to *b, *d, etc."
Traces of the Second Sound Shift, in which voiceless stops become affricates, which become fricatives, can be found in the distribution of modern German dialects. Other topics considered in the chapter on Old High German include the origins of umlaut, modal verbs, nominal morphology, periphrasis and the present perfect, subject pronouns, and loanwords.
"A long tradition has speculated that because Latin (which was known and used in certain circles) had a preterit versus perfect system, OHG speakers took that as a model for creating new distinctions of their own, like the present perfect. This is highly unlikely: the community of Latin-OHG bilinguals was presumably never terribly large, and it would be an unusual kind of language contact change for such a pattern to be transmitted through the whole population. In short, the development of the preterit versus perfect distinction was almost certainly an indigenous development, perhaps supported by non-native patterns."
As well as ongoing sound changes, the Middle High German period saw the "clear rise of plural marking" and "the beginnings of morphological use of umlaut", as well as patterns which foreshadow modern verb positioning. (Though Salmons stresses the importance of avoiding anachronistic reading back of modern forms into the past.) With more texts, it is also possible to say something about variation and its sociolinguistic context.
"What about genitive with prepositions? Early Germanic had prepositions, though few governed the genitive — only one in Gothic, in, which appears with dative and accusative as well. This system is developed in OHG but with some variability. Even in MHG, genitive does not appear widely after prepositions, and the prepositions that can govern genitive all appear with dative or accusative, and they are typically not genitive prepositions today: bî (bei), hinder, (en)zwischen, and many others.
Prepositions that govern genitive today are later creations. Wegen, for instance, was originally von wegen, and appears in the 14th century, according to Kluge, while others like während appear only in the 18th century and anlässlich in the 19th."
With Early New High German, Salmons looks at plural marking of nouns, the pronoun system, modal verbs, and periphrasis, along with aspects of pragmatics and dialects. One focus is on the rise of a more unified language and on early prescriptivist ideas, trying to undermine "the threadbare myth of an inexorable march toward the standard".
"Structurally, the construction of a standard variety is simply an additional layer added to the available forms of the language, and a highly artificial layer. That is, its existence does not replace local dialects, regional Umgangsprachen, or other varieties, and certainly not instantly or automatically. Nor does the presence of a standard halt the ongoing development of real varieties. ... the rise of a standard language is tremendously important socially and politically, but it must be seen in the full context of the full range of German language varieties"
Recent and ongoing changes in sound, morphology and syntax can be seen in final fortition, the pronunciation of mich, rhotics, and the determination of noun gender. These are linked to patterns of regional variation and to concerns about language purity, where Salmons touches on the role of Duden, the dangers of "recency illusion", and fears of new media and of English loanwords.
"Wenn-Sätze were never würde-los and prescriptivists seem to have abandoned the battle. In terms of word order, the establishment of verb-final order, even in printed texts, proves to have come to completion later and in more complex ways than scholars believed not that many years ago.
In other cases, robust change is going on, often along the lines described as grammaticalization. The construction am Xen sein has risen to take a place in expressing durativity, without a strong reaction, e.g. from Duden. A relatively new modal, brauchen, appears to be arising long after the system might have seemed stable, and whole new categories have come to be increasingly encoded grammatically, like evidentiality and politeness."
Salmons touches throughout on aspects of broader linguistics, mentioning for example theories linking regular vowel shifts to language transmission ("a younger generation's non-emphatic vowels appear to be located very close to where an older generation's emphatic ones were"). And he ends with a broad overview of the theoretical ideas he has drawn on. Overall, there is a nice mix of theory and detail. A History of German should appeal to a broader audience than just university undergraduates: to anyone with some knowledge of German and an interest in linguistics.