I'm not sure if this is a publisher promotion or a sale or what, but I noticed that many of Lawrence Block's backlist titles are now priced at $3.99 at Amazon:
Regular HarperCollins pricing for these runs between $6.99 and 9.99 from what I've seen. I checked Kobo, B&N, and Sony earlier and they had the regular prices, just checked
Kobo again and they are now showing the $3.99 prices.
ETA:
These are now available at all the ebook vendors and it is indeed a publisher promotion, which should last a couple of weeks according to Lawrence Block's blog.