Some example code to answer the question of when to move results into a let.

what-to-move-to-let.clj

;; using a let to store intermediat results
(defn recommendations [scores user]
  (let [user-row (.viewRow (:matrix scores)
                           (get (:row-mapping scores)
                                user))
        sorted-by-score (sort-by #(get % 1)
                                 (non-zero user-row))]
    (map (fn [[item, score]]
           {:item item, :score score})
         sorted-by-score)))


;; don't use any let
(defn recommendations [scores user]
  (map (fn [[item, score]]
           {:item item, :score score})
       (sort-by #(get % 1)
                (non-zero (.viewRow (:matrix scores)
                                    (get (:row-mapping scores)
                                         user))))))

Thanks for the comment. I will refactor my code, because this version is very nice.