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The code just after the paragraph

“This construct is so useful that there is a special form called let to make its use more convenient. Using let, the f procedure could be written as”

should be this:

(define (f x y)
  (let ((a (+ 1 (* x y)))
        (b (- 1 y)))
    (+ (* x (square a))
       (* y b)
       (* a b))))

(see http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-12.html#%_sec_1.3.2)