discourse-googlebooks is a plugin for integrating google books with discourse. This plugin has been built to specific needs of site I run ie https://discussbook.com. Nevertheless, I feel that this plugin can be useful for people integrating google books with their site.
Here are some screenshots from the plugin
Screenshots from the plugin
1. Editor integration
2. Category Settings
3. Topic integration. Link to this post 
4. Settings page
Your suggestions and feedbacks are welcome!
I am currently using the modified version of this plugin for discussbook. If you are interested in contributing to development of discussbook, I will be putting the code of discussbook on github in few days, so that people can contribute to the development of site.