This is looking really good! A couple notes though:
I like to use a wholistic approach to cover design. Meaning I design the cover within the boundaries of the book dimensions, the title space, and any other graphic design elements or type that will be a part of the finished cover.
So while this illustration looks great, it's hard to tell if it looks good as a cover if we are seeing it out of context.
The next thing I'd like to see is this illustration with the cover's border around the edges, and the space for the title and type blocked in.