There are two primary audiences for this work. The first is anybody interested in understanding a wide variety of scientific topics. Though not as thorough and wide ranging as Isaac Asimov's science guides, Sheffield writes with the same clarity and and his own style of wit. Even somebody who regularly reads popular science magazines may find some new insight here.
Sheffield delves into the origins of life, subnuclear and quantum physics, possible mechanisms for space travel, physical descriptions of the solar system, superconductivity, viruses and prions, and a lot more including a whole section on "scientific heresies".
The second audience are those interested in writing science fiction, specifically the sort of hard science fiction Sheffield wrote. To suggest story ideas, Sheffield explores some of the borders of modern science where conventional theory gives way to speculation. Along the way, he points out some common traps to avoid when handling topics like near lightspeed travel and suggests specific fiction titles as examples of how a concept has been dealt with. He does not offer any advice on the literary aspects of science fiction or in marketing it. His sole interest is in helping you get your real science right and make your imaginary science plausible.
While the book doesn't have a whole lot about the thought processes of scientists, Sheffield does cover the historical and contemporary objections to some scientific theories, the prejudices that sometimes blind good scientists, and some of the amazing minds that have roamed across several disciplines.
Admirers of Sheffield's fiction will also probably like the asides about its scientific inspiration.
My only objection to the book is that I wish some sections would have had more detail.
The book includes a useful bibliography of fact and fiction titles for further research and an index.