Light Novel, the original medium of the series.
See here for an article listing the light novel volumes.
English No Game No Life Light Novels Edit
Yen Press has released No Game No Life light novels in English under their Yen On label. Currently there are 6 Volumes released and are available via BookWalker (an E-book store), Barnes & Noble (Physical book shop), Amazon (Online shopping service)
The Light Novels can also be read/bought at [second hand] Book stores, Google play services, and free reading websites, such as Wattpad. Though free reading websites is not recommend for it is pirating, and doesn't support the creator or translater(s).