Bomba the Jungle Boy in the Abandoned City is actually a direct continuation of events related in the previous book, "Jaguar Island." Thus far (five books), this is the only such type of sequel. The island realm of Japazy the half breed has just sunk into the Great River and Bomba is trying to escape the fury of both the river and its still spewing (though underwater) volcano, Tamura.
Determined to continue on his quest to locate Japazy (whom Sobrinnini of Snake Island told the jungle boy to find), who was away on one of his trips to Ximotox (the abandoned city), Bomba saves the life of his estranged cousin Giro Gibo, the only surviving tourist of Jaguar Island, discounting Japazy and his party.
The abandoned city itself seems a bit contradictory, in terms of plausibility. It is located on one of the Amazon's largest islands and is decidedly prone to a lot of habitat loss. Yet this enormous and fabulously rich city of Ximitox exists and flourished only a thousand years ago, or so Giro Gibo relates.
After Bomba confronts Japazy, as Ximotox is destroyed, the half breed acknowledges Carson (who is still missing in action from the last book), and Sobrinnini. But not Jojasta, whom Bomba did mention.
Well, sympathetic Japazy falls to his dooming kidnapping by Jojasta instead. In a switch, it is Japazy's diary (of sorts), that provides the opening into the next adventure of Bomba, On Terror Trail.