def < seems to have the wrong symbol: should be
  def < (that: Rational) =
    this.numer * that.denom < that.numer * this.denom

In the code snippet there are 4 operators defined.
The text then states "...three of the comparison operators are defined in
terms of the first one..."
Thus, we are to assume that the first is < and "three" refers to the
latter of the 4 shown operators.
If this is the case then only 2 of the 3 latter operators refer to <. The
last of the 4 operators, >=, refers to >, || and == and not <.

The compare method for Rational is not robust as it can easily overflow. 
The same could be said for the whole of chapter 6.

The last paragraph of the chapter 12.4 mentions "type erasure".
Since this term wasn't mentioned previously in the book it would be
consistent with the rest of the book to add a note/link that "the type
erasure is explained in chapter 15.2" (or a footnote that briefly
explains it).