This book is definitely best suited to people who have had an intro to linguistics already, although such a primer isnt really necessary because the author defines and discusses all terms and concepts before she uses them as tools of analysis. An introductory chapter on phonology, phonetics, morphology, syntax, semantics, and language acquisition from any general introductory linguistics textbook will be more than adequate prep for this book, since it doesn't expound too deeply on linguistic analysis of Japanese; it is exactly what it claims to be, and introduction. The author does accommodate people who have no had no prior experience in linguistics primarily by defining any terminology that's used along with a very brief discussion of the theory, just enough to afford the reader a functional understanding. For example, a phoneme and an allophone are both well defined in the early chapters, and then she uses the sound inventory of Japanese to further illustrate conceptually what phonemes and allophones really are along with defining what the phonemic and allophonic distribution of Japanese sounds happens to be.
There is alot of repetition in this book, which might make it even more so apt for beginning students who haven't yet committed cold to memory all the terms and concepts of linguistic theory. But if your an upper year ling student, you'll definitely be aware of the repetition, even if your not familiar with Japanese. The repetition isn't wholly overwhelming, but actually kind of makes the content memorable so that you wont have to do a second or third reading of most parts.
Looking at graduate level textbooks in mathematics, for example, you'll notice that the expository emphasis is on stringing theorem and proof, leaving little discussion for motivating certain concepts or definitions and not discussing the significance. That may be what is wanted, but this book by contrast very thoughtfully motivates almost every hypothesis or guiding inquiry and slowly works through the process of going from data to conclusion. This is why it is an excellent book for beginner or intermediate level students, for you it will sharpen your linguistic thinking skills.
If your studying Japanese, want to get a more technically succinct understanding, and have no interest in pursing linguistics, then you will still get quite a bit out of this book but probably find it difficult to speed through, and have a grossly incomplete picture of general linguistic theory.
Overall the book is definitely worth buying and studying. There might be better references, akin to the theorem and proof style exposition of math texts, but this one is a good nighttime read for sure.