The art books sucks ass. It's like more of a pamphlet only.
It's only like 5 or 6 pages. The only thing that's awesome in it is a special prequel art work featuring loli Gray in full gear. (All of Stella's Ch.3+ clothes were Gray's old clothes, weapons, booster unit, etc. She even makes a comment about how tight the shorts are)
But most interesting, 2 other girls are with her.