What subjects are located on which floors?

Here's a map of book locations and the five floors of Kennedy Library: https://lib.calpoly.edu/study-spaces-and-tech/maps/

What do the A-Z letters mean?

Each book has a "call number" code designating its address in the building. The call number starts with a letter of the alphabet from A-Z and then gets more specific with numbers.  To view how world of human knowledge is divided up using this A-Z system try Wikipedia's page: https://en.wikipedia.org/wiki/Library_of_Congress_Classification

And here are two highlighted, architecture-related areas: