Here's the basic recap: Mike Mulligan is a steam shovel operator who owns his own steam shovel, named Mary Anne. They have worked together for many years on big, important projects. But times have changed, and now all of the major projects in the cities are being done by gasoline, electric, and Diesel shovels, not the outdated steam shovels. Most steam shovels are being sold for scrap, but Mike can't face doing that to his beloved Mary Anne. Then one day Mike hears that the small town of Popperville is going to build a new city hall, so he and Mary Anne drive out there and offer to dig the cellar, promising that the work will be free if they can't complete it in a single day. One by one, the residents of Popperville turn out to watch, followed by the residents of other towns. All of the spectators motivate Mike and Mary Anne to work very fast, and they complete their job in just one day. But they've forgotten to leave a ramp to drive Mary Anne out of the cellar! After some discussion, everyone agrees to let Mary Anne stay in the cellar, where she will become the furnace for the new town hall, and Mike is hired as the janitor.
This book can be read as having an anti-development bias. It displays a certain amount of sadness for the technological changes that have made steam shovels obsolete. But there is plenty to counter this, unlike in, for instance, The Little House
Perhaps Mike's affection for Mary Anne led him to make some poor economic choices. He could have invested in a diesel shovel instead of taking "such good care of Mary Anne she never grew old." But it can also make good economic sense to keep your capital equipment in good working order as long as possible. This being a children's story book instead of a corporate balance sheet, we don't have the necessary information to decide whether Mike made the right choices or not. But this makes a great discussion topic as your kids get older. And little kids can relate in the right context, too: should we repair this toy or buy a new, upgraded model?
Mike and Mary Anne embody the value of productive work. They are extremely proud of all the work they have done on important development projects. When they are replaced by newer equipment and can no longer find work, "Mike Mulligan and Mary Anne were VERY SAD." (In all caps, no less!) There's nothing worse than being unemployed and useless. Their solution is not to give up, but to prove how useful they can be in the right context. They head out to the small town of Popperville, where it doesn't make economic sense to hire a big diesel shovel, but a steam shovel can still be very useful. They like other people to appreciate their work, too. When they know they are being useful and entertaining to spectators, they work even harder and faster.
The solution reached at the end of the book is pretty much perfect. Even if Mike hadn't forgotten to leave a ramp out of the cellar, turning Mary Anne into a furnace might well be the best economic solution out there. There can't be too many places like Popperville left for very long--economic development will continue to spread from big projects to small ones--and traveling between far-flung little towns with a day's work in each is not likely to be economically viable for Mike and Mary Anne. If Mary Anne can function efficiently as a heating unit, why not? It provides steady, useful work. Mike and Mary Anne are both thrilled with the situation; nobody treats it as a demotion (though perhaps it's a form of semi-retirement, sitting around with visitors telling stories and eating pie every day). Certainly it is much more productive than what other steam shovel operators did, scrapping their machines or leaving them to rust. Mary Anne goes on to have another whole phase of productive life because she was so well kept up by her operator. Mike gets to continue working with Mary Anne, clearly important to him, instead of having to give her up for a different machine. The solution is not only efficient, it also makes everyone happy.
Overall, I think the theme of this book is not nostalgia and anti-development. It's finding a productive niche for yourself given your capacities and limits. Not all of us can be CEOs of Fortune 500 corporations. Maybe we don't have the skills or capacity, or maybe we have other values, like family or leisure, that make other paths better for us. This book is about finding the work you can do, taking pride in it, and finding a situation that gives you the best life possible.