It discusses space flight, from ground-to-orbit rocketplanes, Mars colonization, mining the Moon, asteroids and gas giants, all the way to starflight, in terms of how these things could be done with currently known physics, without resorting to hyperspace or wormholes or similar handwaving. Zubrin does assume our technology will improve: things like deuterium-He3 fusion and matter-antimatter engines do not contradict known laws of physics, but we don't have currently workable engineering for them.
Still, in reading about this sort of stuff, I feel like I'm listening to a bright 16th century natural philosopher describing how we could build airplanes, as incremental improvements over wooden galleons and canvas sails. We don't have *quite* the technology needed to build these things, he would say. He envisions flocks of hundreds of swans in harness, that might lift a galleon into the sky, perhaps to fly it all the way to the Moon. Nothing that violates the laws of physics, of course. This bold and noble project will take decades of work, public support, and a large investment by the Crown, he says. We must start now.
I'm all for starflight. But just as airplanes had to wait on the gasoline engine, I suspect workable starflight will have to wait on a technology, and maybe even a science, that we don't even suspect exists, yet.