Aesthetics are fine, as long as the book and text is readable.

Don't do pictures, do line art. Pictures and shadeing don't really work with the printing most books use.

If you are going to have maps/diagrams/charts. Make sure they are legible. Too many times I have seen fine details get lost because it was too fine for the printing process.

A good example of good maps is the Wheel of Time Series. The hard covers have on their endsheets a beautifully done, full colour, shaded map of the world. But a few pages into the bookblock is (ususally) another map of the world. Showing the same area, but done in lineart. So same details, but perfectly legible. Where as a scan of the full colour version attempted in grey scale would have been a mess.