Now that you've completed this book, you have a great understanding of voice, and text-based chatbots. You're able to build complex, multi-flow chatbots that integrate other services such as S3, DynamoDB, and external APIs. If you've enjoyed learning how to build these systems, then you're in a great position to continue your journey.
There are a lot of different directions you can go with your learning, and I'll outline some possibilities for you—a few specific to Alexa or Lex, and then a few that would be great to learn for both Alexa and Lex.