This might be my third time through this book. We were introduced to it, shortly after it came out, by an interim pastor at the Presbyterian church we attended in Pittsburgh, back in the dark ages when we were still living in academic poverty. Then, I re-read it a decade or so ago, while I was hiding out in my room, during a period of being "sick and stupid". Upon each reading, I've found it to be a great book.
The gist is that the annual Christmas Pageant, which is normally predictable and dull, is taken over by a family from the other side of the tracks, the Herdmans. They are rowdy and obstreperous, and also totally unschooled in all things church. So they ask some rather penetrating questions regarding the Christmas story. Come time for the actual pageant, they somehow manage to convey the actual Christmas story so much more realistically than it is normally portrayed. Through the acting of the Herdmans, we see that the Christmas story isn't about clean, white surbanites in a nice, sweet-smelling, warm stable.