Ex-1.8

(define (cubert x)
  (cubert-iter 1.0 x))

(define (cubert-iter guess x)
  (if (good-enough? guess x)
      guess
      (cubert-iter (improve guess x)
                   x)))

(define (good-enough? guess x)
(< (abs (- (cube guess) x)) 0.001))

(define (cube x) (* x x x))

(define (improve guess x)
  (/ (+ (/ x (* guess guess)) (* 2 guess)) 3)) 

The thing to notice is that due to appropriate use of procedural abstraction only improve(and good-enough a little bit) is changed between sqrt and cubert.