I believe this song is about how women tend to keep things inside (hence the bookend reference) and when nobody is around they let loose and may have a side that they keep hidden from their significant other because of emotional reasons. The song is suggesting that he would like her to open up that side up to him more often, but has not yet discovered a way to remove those emotional "bookends" (that he perhaps gets a small glimpse of at times) without making her feel too vulnerable.
I believe this song is about how women tend to keep things inside (hence the bookend reference) and when nobody is around they let loose and may have a side that they keep hidden from their significant other because of emotional reasons. The song is suggesting that he would like her to open up that side up to him more often, but has not yet discovered a way to remove those emotional "bookends" (that he perhaps gets a small glimpse of at times) without making her feel too vulnerable.