In chapter 17, section 1, I believe an 'if' is missing from the sentence.

The race function returns Left with the output from the first future argument it finishes first, or Right with the output of the second future argument if that one finishes first.

Should read as:

The race function returns Left with the output from the first future argument IF it finishes first, or Right with the output of the second future argument if that one finishes first.