From it ogo:- If it is used in the sense of "I don't want to give you away (grudge)" then it can be either accusative or genitive (мне жаль книгу/книги).
Is the word 'book' being used here just to demonstrate the endings used, or is this another quirky idiom...?
By "I don't want to give you away", do you mean "I don't want to give you to someone else!" or "I don't want to reveal you?" Either way it is a meaning for this phrase that I would not have guessed from looking at it.

Thanks,

Rich B