ehh, you got your facts wrong, these are done by Khang Le, except for the second one, and also that one isn't from Anthony Fransico. Fransico did allot of the character art on offset.
Not necessarily my favorite, but I think it needs to be mentioned here the work of TEAM FULLMECHA (http://fullmecha.com/). Their principal actors right now being JNT (or 'álter egos' JNTHED, Meioh and Nya-NT), the aforementioned huke, and rendat (or raykazuo, more of a pure designer than illustrator like the other two), among other present and past collaborators like Yūsuke Kozaki, for example.
During the last decade they have made an important design assistance for Mr. Kojima's METAL GEAR series. It is in games like the two METAL GEAR AC!D where you can see more of their work without the obvious Yōji Shinkawa's (amazing and powerful) influence:
I honestly don't know about Koki himself, but the term "concept art" is so broad that even fan art can qualify. And the OP didn't clarify on whether it's supposed to be produced in-house at a developer of any sort, so there.
Concept art is for development, is a study of characters, scenarios and objects. It's a useful tool for game designers and programmers to see what they should put in the game. Concept art is art design creation. It's large and complete scenarios which helps making reasonable level design. It's model sheets of characters to study his/her/its movements, expressions, colors, combinations, etc. They are also not only "art", as they have to combine with the design (architecture) of the game and characters personalities/background.
They are sketches and also finished works, but with development purpose.
The rest is illustration.
It is very different to go and buy a concept art book other then buy an illustration book.
As you might tell I prefer concept art books. It's a sad thing that the majority of concept arts and sketches never reach the public.