Heart-labeled religious books.

This is some mystery. These are just some of religious books that I have seen marked with a heart. My grandfather's 1950's Mipi Olelim at the bottom, on top of the modern-day edition.
The books I have checked so far are Talmud, Mishanyos, siddurim. But nobody knows why the heart. Maybe it is something to do with the life in the diaspora, to mark a religious book with a special but non-specific symbol. To avoid the probing eye of an antisemitic authority.