The Apartment Book, by Leo Hartas, DK Publishing, 1995.
I bought this book about eighteen years ago. I put it in my five-year-old's bookcase a year or so ago. My son found it last night. He loved the cut-away building and the story.
I had a lot of fun rediscovering this book, too. Only, unlike eighteen years ago, I had to wear magnifying glasses to see everything. :/