BATS AND A BRANCH

Love it! While you're working on it, give some thought to the positioning of the title text and author name, as well as the backdrop. There's plenty of space for the title and author name in this example, so no problem there, so just think about the backdrop, ie, the shading and coloring. For instance, if the background is really pale and light, you could use very dark text. But your tree is dark and I suspect your background will be too, so the text will probably end up being white or very pale with drop shadow to make it stand out. You have to consider that this white text (especially the author name) will straddle the tree trunk at the bottom, meaning you'll have white text over a very dark tree trunk on the right (good) and also over the "ground" area beyond the tree on the left (fine if the ground area is shaded/colored fairly dark as well).

I have a feeling this image will be very dark in general though, since it's at night — so all dark blues or purples or something? That will work great, and the text can be white, yellow, or some other color that is perhaps picked up in the image somewhere, like the color of his life preserve ring thing.

Also, think about the overall shape/proportions and try to get it with the same aspect ratio as the printed book. I'm sure you've given ALL this stuff thought already, but I know it's easy to forget when getting carried away with sketching...