Well, the book is self-explanatory in the game, you can sell the Merchant 2 items, a dagger and a book but the book comes later on. Has no relevance to the story (in the meantime?) so no spoilers there, it only serves it's purpose to get an erotic scene. Variants is also self-explanatory, they're just... variants, like another outcome or a variation of a scene, no biggie there.
I'll keep the "States" in shadow tho, they're interesting and the game notifies you about them before they happen, currently only for the Mom and two original sisters, Mia (the blonde one) doesn't have a State as of 0.16.2.1, so no worries. PSA, make multiple saves from time to time, don't be a dummy and keep using the same saveslot, consider it as a backup.