An excellent book: ostensibly a beginner conlanger's primer, but it also works quite well as an overview of linguistics itself. It describes all the main areas of a language and illustrates their workings on examples real and imagined.

If you've ever given any thought to audiobooks, you might have noticed that while for most prose they're perfectly fine, they aren't suited for some genres - texts that are math-heavy, have lots of diagrams or important illustrations, and the like.
On the contrary, there are a few books where I think audio works better - if the book is dealing with topics such as accents, pronunciations or foreign languages, hearing the words pronounced correctly beats just seeing IPA on paper.

But for me, this was the first book that I felt really needed both formats: the verbal illustrations of various aspects of language were crucial, but the accompanying PDF came in handy when the audio referred to samples of written script, transcriptions, and so on.