It should be said that not all bookbinding historians find Bennett's argument totally persuasive. He argues, for example, that 'at least 80% of the books offered by the trade [in this period] were sold bound', but if this were true, then you'd expect to find a much greater uniformity of bindings, on different copies of the same book, than you do actually find in practice. Bennett is right, I think, that a customer could walk into a bookshop and buy a book ready-bound over the counter ('one-stop shopping' he calls it -- meaning that the customer didn't have to pay a separate visit to the binder to get the book bound), but the majority of copies would have been stored unbound in the bookseller's warehouse until needed. Anyone who's interested can find a fuller discussion of these issues in Nicholas Pickwoad's review of the book (The Library, Dec. 2005, pp 464-5), Bennett's response and Pickwoad's rejoinder (The Library, June 2006, pp 199-200).

We had long assumed that, for a variety of reasons, books were shipped and sold in sheets ... But all of these were only assumptions, assumptions that had no real evidence to justify them.

I don't know about sold, but there is some evidence of shipping books in sheets being a common practice. An article by C.M. Walbiner describes the efforts of Meletius Karma, the Metropolitan of Aleppo (1572-1635), to have Arabic and Greek books (books of liturgy, the Arabic translation of the Bible and Ficino's Lessico Greco-Latino) and printed in Rome and shipped. In his letter to the Pope, he lists various requirements (the paper should be white and heavy, the letters should not be small, but large, for the benefit of older members of the clergy), including the request to have the books bound. The final requirement is to send 1000 copies of each book ordered and then Karma begs the Pope once again - in Walbiner's translation - "Und um Jesu willen: schick sie nicht ungebunden" ("And for Jesus' sake, do not send them unbound").
Oh and by the way, the Holy See ignored his requests.

This dispute threatens to become contentious. I pray that everyone espects the Languagehat tradition of civility.

Arnold, bulbul: Thanks very much for the supplementary information! I think I learn more from the comments on this blog than I did my last year of grad school.

John: Don't give me that, you snotty-faced heap of parrot droppings!

One thing's for sure: binding a book is a poor substitute for actually hearing how a Nenets word sounds.

Does "bind" mean put a hard cover on the book or just stiching the sheets together?

I ask 'cause I've always wondered why in France new books don't come out in a hardcover edition, rather a format larger than the French paperback, but, still, with only a paper cover (though sturdier). I had always assumed they were merely continuing an older practice of selling books "unbound," which I thought meant without a hardcover.

Before the advent of paperbacks, which were sold ready to read, most French books were sold not only unbound but uncut - everybody owned a paper-cutter (known as a letter-opener in English) for the purpose of cutting the pages from each other as you went. You knew that no one else had read your new book before you, and these books were much cheaper than if they had been bound. Then if you liked the book enough you could take it to a bookbinder and pay to have it bound to your own specifications. Sometimes you found a book in which only a few pages were cut - the reader had started the book but given up on reading it.

Bound books for sale did exist, for instance textbooks for schoolchildren (who would otherwise have destroyed unbound books), or special collections, but the majority were sold unbound. When I first went to England I was surprised to find that all the books were bound. When I see all the remaindered bound books in bookstore bins, I think of the money that could have been saved by not binding them - maybe more of them would have been sold if they had been cheaper.