
I've heard about the first three being seen as one novel before. Are the other books standalones? Or do they form new trilogies/quadrilogies?

Just finished Book #2 of Barsoom; Gods of Mars, by Edgar Rice Burroughs. Loved it, but don't usually like cliffhanger endings. The book was short, and the cliffhanger was done well, so it made me eager for the next one, instead of cursing the author's name for strong-arming me into the next installment.