This book contains some interesting selections but I think the grammatical and vocabulary notes are woefully insufficient for most readers. Considering the geographical and temporal span of this book, it's not surprising that there should be a considerable variety in the styles and grammatical quirks between authors and countries and periods; what is surprising is how little help there is to guide a reader through the knottier sections.
Also, for a reader who is completely unfamiliar with these authors and their social/historical contexts, jumping into the middle of one of their works without a lot of contextual information is just inviting confusion and misunderstanding. For a few of the texts in this book, I had to do a little independent research (a.k.a. googling) on the source so that I could understand what the broader context was so I could better understand what was going on in the Latin in front of me.
So my final review is that it's a fine collection of medieval Latin, but you're only slightly better off this this version than you would be with a version with no notes at all. I'm glad I read it, it was worth making my way through, but if you aren't an expert in medieval Latin, get ready for a job.