Simple, recursive deep-merge in Clojure.

deep-merge-spec.clj

(ns deep-merge-spec
  (:require [midje.sweet :refer :all]
            [util :as u]))

(fact (u/deep-merge {:one 1 :two 2}
                    {:one 1 :two {}})
      => {:one 1 :two {}})

(fact (u/deep-merge {:one 1 :two {:three 3 :four {:five 5}}}
                    {:two {:three {:test true}}})
      => {:one 1 :two {:three {:test true} :four {:five 5}}})

(fact (u/deep-merge {:one {:two {:three 3}}
                     :four {:five {:six 6}}}
                    {:one {:seven 7 :two {:three "three" :nine 9}}
                     :four {:eight 8 :five 5}
                     :ten 10})
      => {:one {:two {:three "three" :nine 9}
                :seven 7}
          :four {:five 5 :eight 8}
          :ten  10})

(fact (u/deep-merge {:one {:two 2 :three 3}}
                    {:one {:four 4 :five 5}})
      => {:one {:two 2 :three 3 :four 4 :five 5}})

(fact (u/deep-merge {:one 1 :two {:three 3}}
                    {:one 2 :two {:three 4}}
                    {:one 3 :two {:three 5}}
                    {:one 4 :two {:three 6}})
      => {:one 4 :two {:three 6}})

deep-merge.clj

(ns util)

(defn deep-merge [v & vs]
  (letfn [(rec-merge [v1 v2]
                     (if (and (map? v1) (map? v2))
                       (merge-with deep-merge v1 v2)
                       v2))]
    (when (some identity vs)
      (reduce #(rec-merge %1 %2) v vs))))

Just for some shameless self promotion:
https://github.com/JasonStiefel/clojure-deep-merge/blob/master/src/deep/merge.clj

Here's one more version.

This one is a lot less concise, but potentially easier to understand. It might also perform better than some other versions, since I'm using loops and recurs.

(defn merge-entries [value-fn into-map entries]
  (loop [result    into-map
         remaining (seq entries)]
    (if-not remaining
      result
      (let [entry   (first remaining)
            k       (key entry)
            new-val (val entry)
            cur-val (get result k)]
        (recur (if (nil? cur-val)
                 (assoc result k new-val)
                 (assoc result k (value-fn cur-val new-val)))
               (next remaining))))))


(defn merge-maps [value-fn & maps]
  (loop [results   nil
         remaining (seq maps)]
    (if-not remaining
      results
      (recur (merge-entries value-fn (or results {}) (-> remaining first seq))
             (next remaining)))))


(defn deep-merge [& maps]
  (let [value-fn (fn [v1 v2]
                   (if-not (and (map? v1) (map? v2))
                     v2
                     (deep-merge v1 v2)))]
    (apply merge-maps value-fn maps)))


(deep-merge {:m {:a 1 :b 2}} {:m nil} {:m {:a 2 :c 3}})
(deep-merge {:m {:a 1 :b 2}} {:m {}} {:m {:a 2 :c 3}})
(deep-merge {:a {:b true}} {:a {:b false}} {:a {:b nil}})
(deep-merge {:a 1} nil)
(deep-merge {:a {:b true}} {:a {:b false}} {:a {:b nil}})
(deep-merge {:one 1 :two {:three 3}}
            {:one 2 :two {:three 4}}
            {:one 3 :two {:three 5}}
            {:one 4 :two {:three 6}})

{:m {:a 2, :c 3}}
{:m {:a 2, :b 2, :c 3}}
{:a {:b nil}}
{:a 1}
{:a {:b nil}}
{:one 4, :two {:three 6}}