thanks for the tip, Beetleking.
It also looks like I should stick to 1 colour for the shadow (and pick one that will unambiguously read as a shadow), and possibly allow some colours to be used as AA.
When comparing my curent books to your edit, the attempt to have "progressive" shadowing simply doesn't work.
Btw, I think I will drop the "diagonal shadows" too, and just have 2 levels of shading, with book-like shape as transition. Hopefully, that should allow me to use the additional tiles to build better textures for the paper and book spines (I'm over-repeating some 8-pixels tile, atm).