The essential solidification of Western Liturgical practice happened in the 9th thru the 19th century. This book explicates the changes and development of both the Divine Office (Liturgy of the Hours), and the Mass during that period. The book is clear, with plenty of references, and makes a confusing topic a little easier to understand. The appendices have very useful information, and the glossary is complete and very useful.
If you are looking for the history and information about the formation, structure and order, content, and the changes in the Divine Office; the bifurcation of the office into Monastic and Secular; the Liturgical year and calendar; the relation of the Roman office to the Anglican version of Morning and Evensong - this volume covers all of these areas.
The book is not concerned with the changes to the Tridentine liturgy that occurred in the 1970's.
I would say this volume in indispensable to the student of this topic, and while I have read several other related works, the clarity of this book is worthy of mention.