[nneonneo]

First, the parent poster was referring to the book being well written, not to the set of practice problems from an entirely different book. Thus, your "horribly written" comment comes off as a non sequitur.

Second, the subset contains k elements. The phrasing "select a subset S containing k of these elements" is standard mathematical terminology. I'm not sure how you read this as meaning that each element needed to be less than k.