Jack lives a kind of ho-hum existence. His family is rich, and he attends boarding school, and he's expected to take over the family business one day. 
 Except he doesn't care about the family business, and would rather dabble in gears and clockwork and create things rather than sit behind a big, stuffy desk. So when Mr. Havelock appears with his parlour tricks and do-dads falling out of his pockets, he's intrigued. And he probably shouldn't have stealthily followed the man halfway across London, but then we wouldn't have much of a story. 
 Jack finds himself transported to Londinium, which is frighteningly similar to his own London--except for the mechanical creatures and the persistent cloud of steam over the city. This city is filthy, home to oddities he's never seen, and he loves it. 
 Except for that little part about Mr. Havelock wanting to kidnap him, but Jacks thinks he can keep a low profile. 
 This is a fun read. Jack meets wonderful people who help him adapt to this alternate world, and he makes some pretty poor decisions that affect everything. I like Jack because he isn't perfect; he's a bratty little kid who wants things his way, and of course his way isn't always right. And I'm partial to the steampunk thing, as you probably know, so the fact he's doing all this while hanging out with a real-life wind-up girl makes things so much cooler. 
 I do wish there was a bit more world-building--it heavily depends on the reader's knowledge of London, and while I know enough to get by I feel like there could've been more. And more differences between the two, and a lot more of the gritty atmosphere. (That's my steampunk bias talking.) But it's a great little story, and maybe Jack grew up a little in his adventures, too. Also a plus.