On page16 of the above book, it show (- 1.5 1/2) returning 1.0.

When I do such in Gambit, I get:

Gambit v4.7.3

> (+ 1/2 1/2)
1
> (- 1.5 1/2)
1.

Here are some other results:

Chicken Scheme

#;1> (- 1.5 1/2)

Warning: cannot represent exact fraction - coerced to flonum: "1/2"
1.
#;2>

Racket

C:\Users\Steve>racket
Welcome to Racket v6.0.1.
> (- 1.5 1/2)
1.0
>

Larceny

Larceny v0.97 "Funny in the Head" (Aug 19 2009 14:35:39, precise:Win32:unified)

> (- 1.5 1/2)
1.0

>

MIT Scheme

(- 1.5 1/2)

;Value: 1.

Petite Chez Scheme

> (- 1.5 1/2)
1.0
>

SCM 5e7

> (- 1.5 1/2)
1.0
>

Why is there such a variety?

Steve