Season 2 will combine the grand finale of the first novel of the series, Leviathan Wakes, with the events of the second novel, Caliban's War.
SyFy have also signed a distribution deal with Amazon Prime, which will see The Expanse launch in several new territories (including the UK and Ireland) in December this year via that service. Season 2 will air on SyFy in early 2017, possibly February.
There are five novels in The Expanse series already available: Leviathan Wakes, Caliban's War, Abaddon's Gate, Cibola Burn and Nemesis Games. The sixth novel, Babylon's Ashes, will be released in November this year.