One of the meanings of 本 is main (which is what we're using here). The main public building is the main building.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
The main building has a classroom for pre-school children.
We are going to make a book that teaches the history of the main building from scratch.
Lots of the people working in the main building of the department store act like someone is forcing them to work, compared to those working at the annex. They have awfully bad attitudes.