A big part of why I've wanted to study Welsh is to get back into the medieval literature and lore about King Arthur. D. Simon Evans's A Grammar of Middle Welsh seemed to offer a unique window into that literature. The book gives a very thorough examination of syntax, grammar, and morphology in medieval Welsh. The emphasis is on examples from extant literature, rather than an idealized presentation of what the language should have been. As a result, I found it a little hard to grapple with at times. Some elements of the language were easy to grasp, from my knowledge of modern Welsh. Others helped to explain oddities of the modern language that used to be standard in middle Welsh. But then there were some aspects that completely disappeared in the modern language that I wish Evans had explained in more detail.
I read the book straight through, but it perhaps works better as a reference. It is divided into numbered paragraphs, making it easy to look up a particular citation. After reading this book, I certainly don't feel like I have complete mastery over middle Welsh, but I at least feel prepared to try reading some of the literature.