I keep coming across new material on commonplace books. Today I Googled Commonplace Books and Libraries. One of the most interesting pages was one at the British Library, http://www.ampltd.co.uk/collections_az/RenCpbks-BL/editorial-introduction.aspx
It begins with a delightful quotation:
"To those, who have been accustomed to the use of a Commonplace Book, the advantage of a convenient Repository of the kind is well known; and to those, who have not, its utility must be sufficiently obvious. The man who reads, and neglects to note down the essence of what he has read; the man who sees, and omits to record what he has seen; the man who thinks, and fails to treasure up his thoughts in some place…will often have occasion to regret an omission, which such a book, as is now offered to him, is well calculated to remedy."
A New Commonplace Book…Properly Ruled Throughout with a Complete Skeleton Index, and Ample Directions for its Use; Equally Adapted to the Man of Letters and the Man of Observation, the Traveller & the Student, and Forming a Useful and Agreeable Companion, on the Road and in the Closet (London, 1799), p.1.
I presume the previous paragraph is the source of the quotation, although it's author remains a mystery as far as I can tell.
I've never read another person's commonplace book, although I've skimmed a few of those that have been printed and the first page or so of an unpublished commonplace book of another reader. But I've yet to read one from page to page as one would read a book.
A friend wrote to me: "To read someone else's commonplace book is to stand at a keyhole and peer into who they are."
What a difficult task that would be. Like another person's letters or journals, however, it is one approach to beginning to understand an individual, at least, insofar as they are not simulating in these forms of personal expression.
Perhaps a person's commonplace book might even be more truthful, since it is unlikely the keeper believes someone else will ever view it. I also think it would be instructive for each keeper to analyze their own commonplace book with an eye to knowing themselves a little better.