The X P on the front of the library building is the Chi Rho. The Chi Rho is the symbol for Christ used openly by Christians since the first century. It is formed by superimposing the first two letters (XP) of the word Christ in Greek.
When the Benedictine monks built the original portion of the library in the late 1950s, the decided to use the Chi Rho on the front of the building, as Christ is the Word, and the library is a house/home for ideas and words.
In the beginning was the Word, and the Word was God. John, 1.