There's quite a long and philosophical preface to this place, explaining why it wasn't published during the war (WW I), and giving Shaw a chance to lay out his views about capitalism, socialism, country society, and a range of other things. It's dated and not entirely consistent, but nonetheless interesting. In theory, it lays out exactly the message of the play that follows, but the play is rather more obscure in its presentation.
The play itself is a mixed bag. The first act is quite funny, in a clever, sarcastic way. By the second act, the jokes and tricks are starting to wear a bit thin, while by the third, Shaw gives up on comedy and ventures into polemic by proxy. Unfortunately, he muddles his argument, and really clouds the waters by suddenly introducing the war, which until the final few pages is entirely absent. This is, according to the preface, part of his point, but as a matter of writing, he presents it poorly.
All in all, amusing and interesting. However, I suggest reading the play first, so that it stands on its own. Then read the preface, and see what you think.