The advantage to doing the previews in the books (rather than in a seperate self-contained publication) is that they're probably slightly less expensive to produce, and less likely to be ignored (and a publisher is less reliant on the retailers to distribute them).

I haven't kept up -- were they doing those for books that were in store at the same time the previews were published, or something that would be coming in another month or so?

In The Back Of Beyond: My photography blog. Everything seen through the camera of my iPhone.