A signed copy, is a book with a simple set of signatures in it, a great way to give us extra support, and have a more special version than the basic book.
A doodled copy, is a standard signature-drawing Erika will do, it won't be a-one-of-a-kind, but it will be hand drawn, and better than a simple signature.
A unique drawing, is a drawing Erika will spend a lot more time on, and it won't ever be drawn twice. Really makes that copy a unique item. She tends to draw all kinds of NSFW things for this tier.