This is a good book. I've been studying Japanese as a hobby for a year or so and making slow but steady progress. I use this book all the time. I love the author's clear, pleasant style, through which I discern an amiable, helpful, knowledgeable person. Reading this book is like being helped with your Japanese by a good friend. I would recommend this book to anyone.
I hope the author does a second edition and alters it slightly. The book's major problem from my perspective is that the author did not decide whether he wanted to write a reference grammar or to write a textbook to teach Japanese verbs.
If it is meant to be a reference grammar, it minimally needs a good index and an informative table of contents. As it is, it is really hard to find anything in the book. Sometimes I know I have seen something in the book, but I am reduced to going through the book page by page to look for it. There is no index, and the table of contents, which consists solely of entries like "Base 1 + nakareba," would only be useful to someone who already knows the stuff in the book. It might be nice also to tighten up organization a bit. As it is, the book is studded with subsections that begin something like "Oh, as long as I'm telling you this, now would be a good time to mention this." So important topics get buried in sections nominally covering other topics. And, since so much in this book depends on the bases from which verb forms are made, it might be nice to have one section where the formation of all the bases is laid out in methodical fashion so that the user, confronted with some utterance like "'you desu' after base 3," can easily track down what base 3 is without paging through the whole book.
However, the author actually seems to have set out to write more of a textbook than a reference grammar because the book generally assumes that the reader knows what has already been covered earlier in it. If that is the case, the author needs to include exercises so that students can actually practice what he is explaining and to give more thought to systematically building up vocabulary and grammar from the rudimentary to the complex as the book moves on. And even then, a decent index and table of contents would help.
I actually think this book would be easier to nudge into the direction of a reference grammar than a text book, and that's what I would advise.
But, please, don't let my comments deter you from buying this book. It has been a great help to me and it will be to you, too.