Having read this book, you now have a great start to working with Twilio. However, where should you go next to keep learning? Take a look:
The Twilio documentation is very complete and includes full details on all of Twilio's different APIs and libraries. You can start exploring them at http://www.twilio.com/docs.
The Twilio Blog at https://www.twilio.com/blog/ frequently posts ideas for projects, help with using new Twilio features, and even general programming advice. It's a great place to follow, if only for inspiration.
If you ever have any difficulties with Twilio, try asking for help on Stack Overflow at http://stackoverflow.com/questions/tagged/twilio. A fair few Twilio staff are often there to help out!