"I want to send her my book."
Translation:Chcę wysłać jej moją książkę.
„Książkę” is the accusative form of feminine „książka”. I believe the accusative case was touched in one of the few tips and notes sections available if you are using the web version.
Otherwise, this discussion should point you to many interesting Polish topics: https://www.duolingo.com/comment/16296174