The City Machine by Louis Trimble is well-written science fiction, suspenseful, tight, with fairly well-drawn characters and understandable motives. It doesn't present any brilliant new ideas, but an original plot combining such ideas as multi-generation star ships to colonize planets, protagonists trying to escape a future city (sort of like Logan's run), a colony comprised of a three-level city, and classes of people corresponding to their level, with rebels in the lower level (slaves, like in Metropolis), plotting a mass escape. The hero may have the key to secrets the rebels need, but while he's recruited and discovers what it's like outdoors, his girlfriend is still in the mid level of the city. Suspense and intrigue ensue.
The blurb on the cover makes it sound a little fantastic, but the author thought through the science; it took the colonists generations to arrive and they chose to live exclusively in a city because they had grown so unaccustomed to the dangers of nature. Not exactly an impossible to put down page-turner, it was nevertheless easy and enjoyable to read straight through.