Quite right, Pat, which is why I said 'I think it should read......'
My reading of Yelnat's 'scenario' is that there is one book (ownership unknown) and there are two groups.
It somehow seems more logical that there should be two groups, each group having one book.
If, however, there were more than two groups, then the sentence could read:
"Is it this group's book or the others'?" with the apostrophe coming after others.
or even
"Is it this group's book or one of the other's?" with the apostrophe coming before the s of others.
although this latter sentence adds words which are not in the original.
You pays your money.....
Incidentally, the only other pronoun I can think of which can take an s plural ending is one.
"I've broken my watch; I'll need to buy a new one."
"I've broken my glasses; I'll need to buy some new ones."
ones, however, is not accepted in Chi.