“More complete that our modern Lectionary — almost sounds like a cross between a hymnal and a LOTH with added readings.
Maybe used in monasteries?”
The Liber is still used today by Traditional Latin Mass (aka Extraordinary Form) groups, as it contains all the liturgical music, and the entire Mass calendar (the TLM uses a different Mass/Feast Day calendar, and works on a 1-year cycle of readings, as opposed to the 3-year cycle used in the New Mass).
It also has a very detailed section giving instructions on the proper singing of Gregorian Chant.
The 1961 Edition of the Liber Usualis is available as a pdf file at the link following.
http://www.sanctamissa.org/en/music/gregorian-chant/choir/liber-usualis-1961.html
Thanks for all the information.