Hugh Howey's Wool series is one of e-publishing's breakout hits. It's also in a format that normally never graces top seller lists: a serial, made up of short, shared-world segments. And it rides a new trend in serials: rather than being tethered to a magazine, Howey publishes independently on the ebook platforms.
Serials hit right in the sweet spot between a novel and a short story. They're quick and digestible, unlike the novel - but world can be returned to, in a way that it can't or won't be with a short story (or at least, frequently isn't; exceptions like Asimov spring to mind).
Publishing independently of magazines is really what makes the new e-serials a new kind of animal. Because they aren't dependent on the magazine to carry it, they're also not tied to specific timetables. An e-serial author could publish every week, every month, more or less frequently, depending on market demand. It puts less Dickenseque stress on things like deadlines and word counts. And readers are only paying for what they want - they're getting their serials a la carte, instead of having to pay for all the other articles a magazine might have. A $0.99 serial e-book is cheaper than any magazine that a reader would buy.
The serial is also ideally positioned on the ebook marketplace. Shorter length means more frequent publishing and lower price points - so though per page 5 serials might cost more than a full novel, there's lower "risk" for the reader buying the $0.99 first part of a serial than the $4 book. Sales for one part of a serial lifts sales for the rest, a more dramatic version of what happens with a book series.
None of this is meant to diminish Howey's work, either, which is well-written and craftily plotted. The world-building is done naturally, the characters are empathetic and multi-dimensional, and the story covers a lot of ground - not a light achievement in the amount of time he takes to do it. And he's successful: he's created a world that I want to return to. I'll be downloading Wool 2 now.