Can't sync books with iPad

I'm able to sync all other types of media, including apps, movies, musics, but just can not sync books. After connecting iPad to my Mac, whenever I check the box in iTunes 9, I get the error message:"Are you sure you want to sync books? All existing songs, movies and TV shows on the iPad will be removed."Why all of my other assets have to be messed with just to be able to sync books?Then I solve