The reviews below make clear that different people have different reactions to this book. Having used a number of beginning textbooks of Persian over the years, here is another commentary that will hopefully put this book into perspective:
It is a very complete and sophisticated handbook to the grammar of Persian, arranged in progressive lessons, but it does in many places demand a knowledge of English grammar (and grammatical terminology) that excees what most readers have these days. Frankly, I think that this book, and particularly the grammatical descriptions/explanations in it, would be very heavy going for a true beginner at Persian, especially someone working on his/her own without a teacher or class.
For the true beginner, or the independent learner, I think the tried and true "Teach Yourself Modern Persian" by John Mace, (and NOT the new book by the same title written by Narguess Farzad) offers the easiest way to become aquainted with the Persian alphabet and language in easy stages in such a way that, even working on one's own, you can make good progress and get a good feel for the structure of the language.
In my view, the Thackston book is a good follow-on book for someone who has already picked up the basics from Mace. Thackston's particular strength, in my opinion, is the way he has analyzed virtually all the aspects of the language and provided very clear-cut sample sentences, always in both Persian and English, to illustrate even the finest nuances of meaning. In this regard, it is outstanding, and the sentence-examples and their translations make the points sink in well even if the explanations are a bit abstruse in terms of grammatical teminology. But unless you already have a basic grasp of the fundamentals of Persian, you may not be able to benefit so much from Thackston.
In short, I think Thackston's book is marvellous as a "follow-on" textbook for people who have already picked up a basic understanding of the way the language works, but is probably a bit heavy for those just starting out. I thus recommend it as your SECOND Persian book, and I think that if you approach it in this way you will value it very highly for the way it clarifies and solidifies what you have learned from Mace.