The idea of that the earth is hollow and has people living inside it, although easily dismissed as crankish, has its roots in two reasonable scientific observations. The seventeeth-century Jesuit and scientific writer Athanasius Kircher, observing volcanic eruptions, theorised that there might be a network of chambers and tunnels within the earth around which molten rock circulated. Later in the seventeeth century the astronomer Edmund Halley, observing the shifting position of the earth's magnetic field, hypothesised that the field was caused by a large magnetic object at the earth's core and that if the field moved there must be a space within which that object could move about also. Both are reasonable observations, flawed only by lack of knowledge about the nature of the earth's core: specifically, about its being molten.
From here on, however, as Standish charts, it gets weird. The next step in the evolution of the theory comes with the American soldier John Cleeves Symmes, who issued a pamphlet in 1818 arguing that the earth was hollow and that entry could be gained via a chasm at either Pole - "Symme's Holes". A ship heading northwards would find itself sailing (the belief that beyond the polar ice lay an open polar ocean is a widely-shared delusion of the time) smoothly round a corner and onto the inside of the earth's surface, where habitable lands, lit by some internal source, awaited exploration.
Standish provides a vivid description of Symmes' theories, their predecessors, and the cultural impact of Symmes and his like. This is a directory chiefly of fiction rather than of cults - the hollow earth is used as a plot motif both by famous authors such as Poe, Verne and Edgar Rice Burroughs, and by others so obscure one salutes Standish for unearthing them. Much of this stuff is low quality to say the least, and Standish plays it for laughs when appropriate; his style is reminiscent of a bouncy college lecturer, intelligent but demotic ("At this point the Professor goes a little batso...."). Although there is some repetition in the various pieces of hack fiction cited, Standish keeps things moving along amusingly. And he unearths some real gems. For me, as for many of the book's readers, I suspect, the real find is Cyrus Read Teed, a pharmacist and physician who changed his name to Koresh (never a good sign) and set up a cult preaching that the earth was concave rather than convex - namely, that the earth is hollow and we live inside it - as well as the usual stuff about the importance of the female principle, living communally in a society dominated by women, and so forth. (Bizarrely, Teed was a distant relative of Joseph Smith, founder of the Mormons, which goes to show how unpredictable historical destinies can be - one of the cousins founds a sect dominated by a peculiar cosmology and rumours of sexual licence, whilst the other [insert punchline here].)
Bizarre cults, hack fiction, a lively narrator - this is fun. It bounces by in no time (and means that Standish has read for you a lot of works that would doubtless be numbingly dull and depressingly bad if you had to plough through them yourself!). Definitely recommended.