Using swap! on a nested map

I'm following Chapter 6 of the book Professional Clojure.

App state is currently defined as follows:

(defonce app-state                                          
  (reagent/atom
    {:projects
     {"aaa"
      {:title "Build Whip"
       :stories
       {1 {:title "Design a data model for projects and stories"    
           :status "done"                                           
           :order 1}                                                
        2 {:title "Create a story title entry form"                 
           :order 2}    
        3 {:title "Implement a way to finish stories"
           :order 3}}}}}))                                            

I need to use swap! to add a new key value to represent a new story, keyed by an id with value of the given fields.

(defn add-story! [app-state project-id title status]     ;
  ; Q. How to use swap! to add a key value pair into :stories?
  (swap! app-state update [:projects project-id :stories] assoc      <- INCORRECT CODE HERE
         (unique) {:title title
                   :status status
                   :order (inc (max-order app-state project-id))}))

The unique function not shown here just generates any unique uuid. The max-order function gets, well, the max order. I had to modify it as the book chapter proceeds inconsistently with the actual final code supplied. Here's my version of max-order:

(defn max-order [app-state project-id]
  (apply max 0 (map :order (vals (get-in @app-state [:projects project-id :stories])))))

Question: How can I use swap! to add a new key value into :stories?

I had a go at it but it beat me for now.

I do get the feeling this nested map is not the best representation - in the final code supplied as a download the author has changed to a more relational-type model with projects and stories both as top level entities, with stories containing a project_id, but would be nice to solve this first use of swap! before moving on to that.