How can I fire a controller action for a particular route or entry?
It's more clear to me now that I've seen the database and extra description from the ticket.

The issue is that you've got an entry that already has has the URI of book.

Normally the code you're trying to use would work, but in resolving a route Craft matches any entry/category requests (Step 4) before it matches any dynamic routes (Step 5). 

When you're testing the action URL directly in the browser that works because it hits step 2.

One way to pull off what you're looking for is to give your plugin a template hook.  Then from the template that loads when book is hit, you can do something like:

{% if craft.request.getSegment(1) == 'book' %}
    {% hook "myAwesomeHook" %}
{% endif %}