Further to my note on Monday about the British retailer WH Smith giving away copies of a Garth Nix novel with every pre-order for the upcoming and last Harry Potter novel, the Nix novel in question is The Keys to the Kingdom: Mister Monday. This is the first novel in the author's fantasy series which is planned to feature seven books in total.
A correspondent informed me that this is the next step in an evolving book marketing cycle which has also featured book chains selling "bespoke" editions of some books, ie with specific and individual book covers.
I wonder if the next step for these chains is to sell combined editions of several books in a series, or back-to-back publications like the old Ace doubles of the 50s and 60s. You can expect some interesting volumes on sale when the print-on-demand technology becomes widespread.