entries.js

import Ember from 'ember';

export default Ember.Route.extend({
  model() {
    return this.store.findAll('entry');
  },
  isExpanded: false,
  actions: {
    expand() {
      this.controller.set('isExpanded', true);
    },

    contract() {
      this.controller.set('isExpanded', false);
    }
  }
});

index.hbs

{{#each model as |entry|}}
    <h2><a href="/entries/{{entry.id}}">{{entry.title}}</a></h2>
    <p>{{entry.word_count}} words</p>
    {{#if isExpanded}}
        <div class='body'>{{entry.content}}</div>
        <button {{action 'contract'}}>Contract</button>
    {{else}}
        <button {{action 'expand'}}>Show More...</button>
    {{/if}}
{{/each}}

I believe that in this case the problem comes from your isExpanded property.
You have that property referenced in your route when your template can only access properties from this same route's controller.
So I see two solutions:

• use a controller for that route
• instead of a controller use a component <- preferred solution
  If you haven’t used a component before I recommend to start from here https://guides.emberjs.com/v2.2.0/components/defining-a-component/