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 special version of the basic book.
A doodled copy, has a standard signature and drawing Erika will do, it won't be a-one-of-a-kind, but it will be hand-drawn!
A unique drawing, is a drawing Erika will spend a lot more time on, and it won't ever be drawn twice. It really makes that copy a unique item. Erika tends to draw all kinds of NSFW things for this tier, as well as a lot of vegetable drawings.