I have a book on my bookshelf, Eric Foxley's Unix for Super-Users. It was published in 1985, and it answers this question on page 52, the first page listed for the entry 'lost+found' in its index.

This is surely not the earliest book mention, is it? (It'll be in earlier man pages, of course.) Google Books does not give me an earlier one, although it does yield another 1985 book.

Fun fact: Foxley cautioned that lost+found must be pre-sized ahead of time, because the fsck of the time did not grow the directory to fit found files.