The Long Earth and it's sequel, The Long War are both $1.99 for ebooks (at least on Barnes & Noble, I haven't checked Amazon) Amazon too. The Long Earth is amazing, I haven't read The Long War yet but it's been on my tbr pile for awhile.
Blurb for The Long Earth:
The possibilities are endless. (Just be careful what you wish for. . . .)
1916: The Western Front. Private Percy Blakeney wakes up. He is lying on fresh spring grass. He can hear birdsong and the wind in the leaves. Where have the mud, blood, and blasted landscape of no-man's-land gone? For that matter, where has Percy gone?
2015: Madison, Wisconsin. Police officer Monica Jansson is exploring the burned-out home of a reclusive—some say mad, others allege dangerous—scientist who seems to have vanished. Sifting through the wreckage, Jansson finds a curious gadget: a box containing some rudimentary wiring, a three-way switch, and . . . a potato. It is the prototype of an invention that will change the way humankind views the world forever.
The first novel in an exciting new collaboration between Discworld® creator Terry Pratchett and the acclaimed SF writer Stephen Baxter, The Long Earth transports readers to an infinity of new worlds. All it takes is a single step. . .
Blurb for The Long War:
War has come to the Long Earth. . . .
Humankind has spread across the new worlds opened by explorers Joshua and Lobsang a mere decade ago. Now "civilization" flourishes, and airships link the multiple Earths. A new America christening itself "Valhalla" has emerged more than a million steps from the original Datum Earth. Like the American revolutionaries of old, the Valhallans resent being controlled from afar. And in the wake of humankind's advance, the trolls—graceful, hive-mind humanoids whose song has suffused the Long Earth—are beginning to fall silent . . . and disappear.
Joshua, now married and a father, is summoned by Lobsang. It seems he alone can confront the perfect storm of crises that could plunge all of the Long Earth into war—a war unlike any that has come before.