What sorts of books?

A new acquaintance and his library. From Anecdotal Evidence:

Samuel Johnson by Joshua Reynolds

.... On this date, April 18, in 1775, Dr. Johnson, Boswell and Sir Joshua Reynolds were visiting Richard Owen Cambridge at his home on the Thames, near Twickenham.

Boswell, who recounts the meeting in his Life, was already acquainted with Cambridge, whom he introduced to Johnson in the library. With pleasantries out of the way, Johnson “ran eagerly to one side of the room intent on poring over the backs [spines] of the books.” .... Cambridge, by all appearances a cultivated and courteous man, says to Johnson:

“Dr. Johnson, I am going with your pardon, to accuse myself, for I have the same custom which I perceive you have. But it seems odd that one should have such a desire to look at the back of books.” Boswell observes that Johnson, “ever ready for contest, instantly started from his reverie, wheeled about and answered”:

“Knowledge is of two kinds. We know a subject ourselves, or we know where we can find information upon it. When we enquire into any subject, the first thing we have to do is to know what books have treated of it. This leads us to look at catalogues, and at the backs of books in libraries.”

Something else was going on. Many of us, when visiting, examine our host’s book shelves forensically. Is this the library of one who reads broadly and deeply, and with taste? The mere presence of books, even many books, means little. What sort of books has our host accumulated? Do they appear to have been used enthusiastically and often, or are they interior decoration, put up to impress credulous guests? ....

When visiting acquaintances have you ever evaluated them based on the content of their libraries (or the absence thereof)?

Anecdotal Evidence: `Through Your Body in an Instant'