Added Books section under resources (#183)

Stjepan from Manning here. I thought this title might be a good match for your list. Since there wasn't a "Books" category I created it myself. Thank you for considering it.

README.md

@@ -34,6 +34,7 @@ Awesome Wagtail [![Awesome](https://cdn.rawgit.com/sindresorhus/awesome/d7305f38
   - [Presentations](#presentations)
   - [Podcasts](#podcasts)
   - [Videos](#videos)
+  - [Books](#books)
   - [Showcases](#showcases)
   - [Lists](#lists)
   - [For Editors](#for-editors)
@@ -341,6 +342,10 @@ Awesome Wagtail [![Awesome](https://cdn.rawgit.com/sindresorhus/awesome/d7305f38
 - [Wagtail Wednesdays #07 - How to Enable the Wagtail Styleguide](https://www.youtube.com/watch?v=_CfU9UivYPI) - It’s a really helpful resource that takes no time at all to enable and it allows you to check your components against the guidelines and shows all the available Wagtail icons.
 - [How to Deploy Wagtail to Google App Engine](https://www.youtube.com/watch?v=uD9PTag2-PQ) - Focus is Google Cloud Platform but a great introduction on how to get Wagtail up and running in their PAAS.
 
+### Books
+
+- [Wagtail CMS in Action](https://www.manning.com/books/wagtail-cms-in-action) - Book about creating powerful, simple web applications using the Wagtail content management system.
+
 ### Showcases
 
 - [Made with Wagtail](https://madewithwagtail.org/) - A showcase of sites and apps made with Wagtail CMS.