Frontier have (finally) made the digital art book available, contains a good amount of concept art, some info about their design process and developement, and their future plans. If you enjoy all things sci-fi it's well worth a browse.
https://drive.google.com/file/d/0B6KEY4QJTmolV3Zid1F2bzRmZ3M/view?usp=sharing
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Thank you !
Have fun