Pratchett, Terry, and Stephen Baxter. The Long Earth
Summary: In 2015, plans for a device which allows anyone to step from this Earth to parallel Earths are circulated on the Internet and almost everyone starts 'stepping' sideways.
Series note: A sequel is anticipated.
Comments: In a sense not really alternate history, as there is essentially no historical divergence between the parallel Earths and ours. Ours, in fact, is the only one populated by homo sapiens.
Published: Harper 2012 (0062067753BUY), Doubleday UK (0857520091).