Originally published as part one of his History of Greek Philosophy, v.3: The Fifth-Century Enlightenment; part two has also been published separately as Socrates, which I will be reading in a month or so. I read the first two volumes of the History, on the Presocratic natural philosophers, almost fifty years ago; they are still the best books I have read on the Presocratics. Like the Presocratics, the Sophists are known only through fragments and generally hostile references in Plato and Aristotle, and even more than in the first two books this is a speculative attempt (though based on what evidence we have) to reconstruct their thought.
As the subtitle of the original version indicates, the Sophistic movement had many similarities in themes and spirit with the Enlightenment of the late seventeenth and eighteenth centuries, and Guthrie points them out. He makes the point that there was no real "school" of Sophists, and they differed on most points among themselves; what makes them an identifiable group is not the answers they gave but the questions they were concerned with, many of which were first raised in the fifth century BCE and are still being debated today. These thinkers are far more interesting to me than the more conservative Plato, and it is really unfortunate that their works have been lost; Guthrie suggests that it was because they were more topical than systematic writers and that Plato and Aristotle more or less superseded them, but I think it is also due to the fact that all ancient writers had to pass through the bottleneck of copying by the Christians and Moslems in late antiquity and the Middle Ages, and these writers were simply not as congenial as Plato and Aristotle to the religious-minded. Guthrie unavoidably has to discuss the Sophists largely on the basis of the use of them as characters in Plato's dialogues, which probably gives a fair idea of what the movement as a whole was like but is very unreliable when it comes to the specific positions of particular figures.
Most of the book is organized by themes, such as the Nomos-Physis (Convention vs. Nature) opposition, the idea of the "Social Compact", Ethical Relativism, Rationalizing of Religion and Skepticism, etc. At the end he briefly discusses the ten individuals we know the most about, Protagoras, Gorgias, Prodicus, Hippias, Antiphon, Thrasymachus, Critias, Antisthenes, Alcidamas, and Lycophron, and tries to reconstruct their positions as coherent approaches. Not surprisingly nearly all of these figure as characters in Plato's dialogues.