From such small acorns, etc.: Edgar Rice Burroughs managed to generate a seven-novel series set in Pellucidar, the world he imagined inside the Earth's hollow centre, and it all started with little At the Earth's Core. Not even 160 pages, and with only about a half-dozen feature characters, At the Earth's Core is an awfully slim volume on which to build a multi-volume superstructure, but you know what? I'm happy with it, because a paradoxical virtue of pulp fiction is that it has to both (a) stay humble and not try too much, and (b) reach for the fricking STARS.
With At the Earth's Core, as he was to do later with Back to the Stone Age, Burroughs showed a real talent at telling a small-scale story inside a large-scale opera. Here, the role of "small-scale story" is played by David Innes' all-American love for Pellucidar's Dian the Beautiful, and the opera-scale story is Innes' discovery (with Abner Perry) that the Earth is hollow rather than solid, as well as filled with plant and animal life, including multiple species that seem to have developed intelligence through separate evolutionary tracks. In Back to the Stone Age, the small story is Von's attempt to survive in Pellucidar (and to shed the rest of his name, Friedrich Wilhelm Eric von Mendeldorf und von Horst) while earning the love of La-ja, so yeah, there's kind of a formula, but it's something that Burroughs does really well in these two novels (and quite well in Tanar of Pellucidar, too).
I'm not too sure, though, how Burroughs' publishers decided that At the Earth's Core would sustain a series after it. Sure, Burroughs left all kinds of clues that there could be a whole raft of gripping yarns following on from here, but what makes a publisher pick one possible series rather that another? Were they just giving the author of Tarzan and John Carter the chance to make them some money in an additional series? After all, Burroughs produced 11 John Carter books in 30 years (5 in the first 10 years), and 24 Tarzan books in 38 years (6 in the first 5 years), so maybe this just seemed like another horse they could ride, since Burroughs had already started two profitable series before this novel was published, but it's hard to know without digging into surviving correspondence between Burroughs and his assorted publishers.
Incidentally, even though I've read only the first paragraph of Savage Pellucidar, the final novel in the series, and I can't see myself reading Land of Terror given the summary of it, I'm calling the competition right now: hands down, the best novel in the series is Back to the Stone Age. I mailed a copy to Harold Rhenisch, since he'd shown a genuine affection for Zane Grey's similarly out-there Ken Ward in the Jungle, so hopefully I'll hear one of these days how he felt about it....