Do mangakas usually release art books for their series with clean cover art, concept art, etc. after the series itself is finished? I can find a lot of the original cleaned pieces used for covers of classic stuff like popular Jump shit and even something really niche like Trigun, Guyver or fucking Five Star Stories, but not counting mangakas that work as both comics artists and illustrators (Posuka Demizu, Obata, etc.), I can't find almost nothing for recent series like Kengan Ashura, Vinland Saga, Usogui, etc. other than the covers themselves and occasional color spreads, both of which still have text in it. Why it is like that?