Immensely helpful - a great overview of medieval (high to late) manuscripts in French, their owners, production, and a host of other useful information pertaining to manuscript creation, reading, sharing, etc.
It is an overview, however, so it's useful for mining down into the offered examples in your own research. Busby does not go into huge amounts of detail about each example (because the scope of the study would not allow that), and he hasn't used primary archival material and sources, but relied upon the scholarship of others - this must be remembered too when using this book.