Broken Universe is the sequel to alternate universe novel The Walls of the Universe. In The Walls, Farmboy John's doppelganger (himself, from an alternate universe; John Prime), gave him a device that would allow him to move from one alternate universe to another (the universes being identical but for different decisions made, with small or large consequences), but wouldn't allow him to move back, including to his home. The Walls ended with Farmboy John figuring out how to build a gate of his own to allow backward travel between the universes. Farmboy John, Grace, Henry, and their doppelgangers begin to use the device and its replicas to begin building a cross-universe business empire. Complications predictably ensue.
I can't exactly say that Broken Universe picks up where The Walls of the Universe drops off, because it changes the last scene of The Walls, without explanation and without need. Presumably it was done to inject some oomph into Broken's open, but it was jarring to read directly after finishing The Walls. It's also just lazy writing; Melko could have easily created the desired emotion without rewriting his earlier work. This is, unfortunately, a significant complaint for Broken. Plot, logic, and scientific holes abound.
The first half of Broken is, like the middle third of The Walls, heavy on the nuts-and-bolts of cross-universe business. I think I enjoyed it more than most in The Walls, given my general interest in business and in entrepreneurship in particular, and it continues to raise interesting questions (Is there anything wrong with stealing an idea from one universe and making money off of it in another? After all, they're creating value in the new universe by giving them something they presumable want and would not otherwise have, and they're doing it using technology only they have.), but much of the suspense of The Walls is lacking. When antagonists arrive offering threats to more than their business interests, they're somewhat disappointing. On the other hand, the action sequences in the second half of the book are pretty good, and Melko doesn't pull any punches.
I would recommend Broken to anyone who thoroughly enjoyed The Walls. If you finished The Walls unsure whether you wanted to continue, you can probably live without Broken. Melko does answer a lot of the questions left from The Walls, albeit not all, and there is no obvious sequel hook. He also explores a lot of the themes of The Walls and implications of alternate universes in greater depth.
Disclosure: I won an ARC of Broken Universes through a Tor.com sweepstakes.