A sure sign that a book is edging closer to publication is the arrival of the Advance Reading Copy. The public do not often get to see ARCS. They are published in reasonably small quantities for booksellers, reviewers and librarians and come out about six to eight weeks before publication.
ARCS are also incredibly collectible and when I was a bookseller, I had clients who specialized in First Editions and either Proofs, Galleys or ARCS. At the moment an ARC of the Bloomsbury edition of Harry Potter & The Prisoner of Azkaban is currently being sold for over $41,000.
ARCS are not for sale and each copy is clearly marked as such. (But of course they end up on sale.) It will be interesting to see how the explosion in e-publishing will impact on ARCS. As more and more of the business moves to digital, I can see a day when electronic versions are sent out rather than expensive printed copies.