(define power
(lambda (b e)
(if (= e 0)
1
(if (even? e)
(let ((p (power b (/ e 2))))
(* p p))
(* b (power b (- e 1)))))))
and the logarithmic iterative version:
(define power
(lambda (b e)
(define power-product
(lambda (a b e)
(if (= e 0)
a
(if (even? e)
(power-product a (* b b) (/ e 2))
(power-product (* a b) b (- e 1))))))
(power-product 1 b e)))