I've bought a few books on grammar to help me in my quest to learn both spoken and written Welsh and this is the one I've found to be the best so far.
The context of my learning experience is that I attend a course on a weekly basis. The sessions are good but, as with all classes for adults learning Welsh, the emphasis is on speaking Welsh and not on understanding grammatical points. As a grammar geek this doesn't meet my personal needs.
I understand that with some aspects of languages you just have to accept what you're told and go with the flow ('red car' in English becomes 'car red' in Welsh - that's it, no arguments!). However, whenever possible, I need to know the rules and the exceptions to the rules - this book explains them.
It does it in a very clear manner. Each chapter deals with an aspect of the language (pronouns - adjectives - verbs etc.) so I find it very easy to find the right chapter when I want to check something. I find the chapters interesting and often read one from beginning to end, simply as educational reading material. There is always a very clear explanation of the specific, grammatical point and this is made even clearer by brief sentences in English and Welsh illustrating everyday usage of the point.
I think the combination of attending a course and personal reading is essential. The weekly sessions and the book are mutually beneficial. The book alone would explain the grammar (which is its aim) but would not help me to speak Welsh. However, I would probably have given up the classes if I hadn't found this book (and another I have also reviewed) as I was so frustrated by not knowing and understanding the theory that underpins the practical use of the language.
My favourite quote is when the writer states that a word "suffers a mutation" - I wonder if that was a Freudian slip or a subtle gesture of solidarity with the people for whom the dreaded mutations cause so much suffering!