"Skeleton Men of Jupiter" isn't a separate book in the edition I used, although it is sometimes published as one.
In the edition I used, "John Carter of Mars" contains two novellas:
"John Carter and the Giant of Mars"
"Skeleton Men of Jupiter"
Hence no separate TOC entry for "Skeleton Men".
I've double checked with two different editions of "Skeleton Men" and they both finish in the same place - after 9 chapters. I agree that it is a somewhat "abrupt" ending, but that appears to be the way that it really does end. It is only a short short, or novella perhaps, not a full-length book.
What do you believe is missing from it?