consC in Seasoned Schemer

Question

The seasoned schemer on page 131 and 132 has the below definition of consC and deep.

(define consC 
  (let ((N 0)) 
   (lambda (x y) 
    (set! N (add1 N)) 
    (cons z y))))

(define deep
  (lambda (m)
    (if (zero? m)
      (quote pizza)
      (consC (deep (sub1 m))
        (quote ())))))

I have a problem that when deep use consC each time to recursion,why (let ((N 0)) don't reset N to 0?