And do you mean like a textbook of music theory and/or composition? Well I do have Benner Theory for Piano Students which is a set of five relatively short books which could help get you started, but you might wanna ask some of the other musicians (GIRaka, Kurokun, Jethawk, rtnario, Uncy Dave to name a few) to see if they might recommend anything better.
For that, you might consider adding an optional "easy mode" with more health. For me for example Dragzeloid`s kitchen is no problem, but for a guy who is new to Cave Story it will be.
Hey! I was thinking about Wrath of Revorshkenahl. It is a finished, nice mod so why not upload it at freeware game hosting sites and similar? It would be nice that more people outside of Mirai got acquainted with WoR, I think they would love it.