swimlanes.io sequence diagram plugin
Add the below to your book.json file, then run gitbook install :
{
    "plugins": ["swimlanes-io"]
}Usage
{% swimlanes %}
  Title: Simple example
  A -> B: Ping
  B -> A: Pong
{% endswimlanes %}
By default an "open in editor" link will be shown at the bottom of the diagram. 
To hide the edit-link set the `show_edit_link` argunmnent to "false".
{% swimlanes show_edit_link="false" %}
  ...
{% endswimlanes %}  
See swimlanes.io for more details