There isn't really a current API guide, but I've found studying Michael's code (particularly the .js and .htm files in MoI's commands directory) to be quite helpful in developing a sense of how the API works. The v3/4 release notes also contain numerous hints about scripting and the API as do the forum posts.

v3 release notes: https://moi3d.com/wiki/V3Beta
v4 release notes: https://moi3d.com/wiki/V4Beta

--Larry