The big themes I saw:
– Slavery
– Evil is as evil does; not by virtue of being
– Human vs. Demon
– Is Tom becoming evil because he has turned into a demon?
Symbols…
Chains, marionette?
Don’t know about unifying themes from book 2 or book 3 or book 4. Would need a synopsis.
Maybe each cover should show the central conflict from each book? Not always a scene mind you. A central conflict in Into the Abyss is truth vs. belief.