Despite the book's subtitle, The Day the World Exploded: August 27, 1883, comparatively little is about the eruption itself. Winchester, who was a geologist before he became a journalist, goes into depth on the geological, ecological, and political history of the region surrounding Krakatoa, which lies in the Sunda Strait, between Sumatra and Java (or rather lied, until the eponymous date when it blew itself up and crumbled into the sea). Most complaints I've seen about the book have been in regards to Winchester's digressive narration, but this is in keeping with one of the major themes in the book, which is Krakatoa's position as one of the first truly global modern events. Global here being used in the McCluhan "global village" sense, as news of the eruption spread worldwide in hours via underseas telegraphy. The effects of the eruption are as important to the story as the eruption itself.
More than just emphasizing the global nature of the event, Winchester's digressions tend to focus on the tangential but necessary results of the circumstances that made the eruption possible in the first place, from the strange biodiversity of the Malay archipelago (to the west of the Wallace line the islands are exclusively inhabited by Asian flora and fauna, while to the east is exclusively Australian. At their closest, these islands are only a handful of miles apart. This odd ecosystem is the result of one tectonic plate moving west from Australia and another moving east from India) to the mythology of the native Indonesians. But Winchester also likes to point out the neat coincidence, the way things affect or merely reflect each other. Krakatoa wasn't just a volcano that erupted one day. The eruption, what led up to it, why it was so well recorded, our attempts to understand it, one of these stories cannot be told without the others. Winchester's approach to history is not one of discrete events occurring in sequence, but of thousands of events, happening simultaneously, all, to some degree, affecting each other. Digression, then, is not a foible to be forgiven, but a necessary trait of this kind of history.
Of course, those turning to Krakatoa primarily for descriptions of the eruption itself and its immediate aftermath will be disappointed. The book is more accurately about the total history of the island of Krakatoa, not just "the day the world exploded." As such, everything from movements of the lithosphere to Dutch colonialism need to be addressed. Nevertheless, Winchester manages to consistently bring the story back to the titular volcano as it geared up for its big day.
If you're the type of person who likes to hop from idea to idea, and discover connections between disparate subjects (even if the connections are merely semantic), this is the type of history book that will be right up your alley.