In the HTML version
(http://www.artima.com/pins1ed/functional-objects.html) I can read in the
first sentence:

A rational number is a number that can be expressed as a ratio n/d, where
null and null are integers, except that null cannot be zero. null is
called the numerator and null the denominator. 

Please correct the "null".

Thank you

[FIXED (Thanks!)]

Right at the top, under 6.1, it says:
"A rational number is a number that can be expressed as a ratio n/d,
where null and null are integers, except that null cannot be zero. null
is called the numerator and null the denominator."
I don't believe the "null"s here are intended ;)

[FIXED (Thanks!)]

The lessThan method will yield the wrong result if exactly one of the
denominators is negative.

In listing 6.5, the add (+) and subtraction (-) method definitions where
the parameter is an Int, it occurs to me that in keeping with DRY might
be better written as:

def + (i: Int): Rational = this + new Rational(i)

and

def - (i: Int): Rational = this - new Rational(i)