We replaced a dozen or so titles as a reward, then topped off again to a few more books than 100, mainly to include topics we could not bring ourselves to omit. These groups of books and their names are our own doing. They name categories that we slowly recognized within the motley selections. By no means sharp-edged, each denotes a center around which friends can gather, perhaps more like at a picnic than a rugby scrum. The terms we use are not normative but descriptive, for the list itself suggested the categories and not the other way around.
Books have distinct purposes, hence rather distinct readerships.
The full list includes a few textbooks. Then there are many books, usually rich and deep but demanding prepared readers. Perhaps it is a dialect of mathematics that readers need to know, perhaps a detailed technical analysis of lengthy and essential data like forms and counts of bones in paleontology that try general readers' patience. We call these monographs. But in every discipline the list holds many more books of highest value, usually shorter and always less technical in detail than the fraction we mark as monographs.