No prior knowledge of linguistics is assumed, with the theory used explained as it is introduced. Enough of the International Phonetic Alphabet to represent the sounds of German. Some basic "phrase structure" grammar. Aspects of the theory of tense, modality, and voice. And so forth. There's no room to go into great depth here, but notes provide extra details and references and the result would be quite a nice, if challenging, way for a novice to linguistics to get a broad introduction to the field.
The theory that is used is kept closely tied to the specific features of German. Here, without attempting to be comprehensive, Fagan goes into some detail. German phonemic contrasts. The major historical shifts in consonant and vowel pronunciation. The broad geographical structure of contemporary dialectical variation. Attempts to address gender equity and sexist language use. Among much more. Some of this connects fairly directly with language learning: much of the material on morphology, most obviously. Other material helps to understand the structural or historical logic behind apparently arbitrary aspects of German: vowel changes in different inflections, for example, or the polite forms of address.
German: A Linguistic Introduction is designed for use as a textbook by university students studying German, and includes a small number of exercises at the end of each chapter. It could also be read by anyone else using or learning German, however, or, since it doesn't assume a knowledge of the language, by those with a more abstract or linguistic interest.
Addressing the same market, there are a range of alternative books covering similar material, including The German Language Today: A Linguistic Introduction (Charles V.J. Russ, Routledge 1994), The German Language: A Linguistic Introduction (Jean Boase-Beier and Ken Lodge, Blackwell 2003), and The Structure of German (Anthony Fox, 2nd edition, Oxford University Press 2005). I chose German: A Linguistic Introduction because it has chapters on the history of German and regional dialects and because it has glosses for all the examples, which as a language learner I found useful. Fagan doesn't cover stylistics, spelling and spelling reform, or pragmatics, for which I read the chapters on those in (respectively) Boase-Beier and Lodge, Russ, and Fox.