While I am playing as a Maxwell I want to create as many puppets as possible but the black area is covering my sanity points. Is there a way to get rid of this black area? I know that I can kill puppets, but I am wondering if there is such an item which will remove that black area as like booster pack does to health.
There is no way to increase your maximum sanity on Maxwell. The only character who can do that is WX-78 depending on his upgrades. Source
There is no way to regain the sanity sacrificed with the Codex Umbra safe for removing the shadow-creatures summoned by it.
If you really want to get your sanity up again I guess you could use the console command
c_setsanity(percent), percent being fractal, meaning 90% = 0.9. I don't know what happens if you set a higher percentage than currently possible due to the Codex Umbra as it is undocumented and I don't have time to try it out myself at the moment. This site provides detailed instructions on how to use the command console in case anyone is interested.