Books with adjectives like "fast-paced adventure-filled thrill-ride" are usually overhyped. Usually. In this case, The Walls of the Universe deserves such tags. It is fast-paced, adventure-filled, and thrilling. I finished it in a day--and it's a fair sized book--because I had trouble putting it down.
All of the main characters, with the possible exception of the villains, are complex. Melko sets up the story so we at first believe it will be John Rayburn versus … John Rayburn. Another John--called John Prime for the sake of sanity--comes to "our" John's universe and tricks John into taking Prime's universe-hopping device on a test-drive. This is a trap, since the device is broken and can only move forward across universes; John can't go back, and Prime gets to assume John's life there. With this scenario firmly in place, The Walls of the Universe casts Prime as the remote antagonist and John as the stranded hero, struggling to get back his universe and his life.
Instead, the plot turns out to be much deeper, and Prime himself is more than a one-sided antagonist. In the end, Prime aligns himself with John against the more dangerous villains, although he continues to act in a morally questionable manner. John contemplates several acts we would also consider morally questionable has he debates what he'll do now that he's stranded from his own universe.
Some of the minor characters, such as Bill and Janet Rayburn, and Ted Carson and his father, are fairly stock. Good guys, bad guys, move along, nothing to see here. However, this aids Melko's set-up, since we eventually learn that there exist in the multiverse "singletons"--people who naturally have no duplicates in other universes. By portraying some characters as identical or similar in every universe in which John encounters them, Melko emphasizes the "duplication" aspect, which becomes a key part of the story when John confronts the villains.
In a somewhat original twist, Melko's established a cross-dimensional trade on Uideas*; Prime (and then, reluctantly, John) introduces a popular invention from his universe that's lacking in whatever universe he currently resides, such as the Rubik's cube, SCUBA gear, pinball, etc. This isn't a simple castaway's tale--there are other universe-hoppers, most of them exiles who will stop at nothing to get home. John encounters one such group, drawing attention to himself with the invention of pinball, when he settles down in another universe and attempts to understand the device he has. We're given the impression that portable devices are extremely rare--to the point that some travellers don't even believe they exist or are feasible. Once the villains learn that John has one, and that he isn't an exile or singleton has they believe, just a farm-kid, all hell breaks loose.
The plot never stops advancing, and the dialogue is pretty fun: "You … mulched him?" being my favourite example. Maybe it's just because I'm a science fiction junkie and am fascinated by the concept of multiple universes, but Melko manages to draw me into the book and keep me there. I love watching how John interacts with multiple copies of people--exemplified by his increasingly irate conversations with Dr. Wilson the physicist--and observing the contrast between John and John Prime. Melko uses the concept of universe-hopping to its full potential, making for a ripe story.
The only part of the book that disappointed me was the dearth of details related to the builders of these devices and the reason so many people get exiled. We get hints throughout the story, of course, including something about people called "Primes" (presumably not related to John Prime) and "paths," but no clear explanation. This has the benefit of keeping the story moving without slowing us down, of course, and I appreciate Melko's pacing abilities. However, it's somewhat exasperating to be kept in the dark! It looks like this is just the first book in a series, fortunately, so we should be learning more.