Conversational design seems like a brand new design discipline, however it's actually been around for a while. If you're creating conversational experiences, here is a partial list of lessons and wisdom to help your efforts.

Designing Voice User Interfaces

Wired for Speech: How Voice Activates and Advances the Human-Computer Relationship

So you want to be a voice designer

Voice UI Best Practices: Learning from Phone Systems

Voice & Gesture

Designing for Actions on Google Assistant

Alexa Skills Kit Voice Design Guide

The Conversational UI and Why It Matters - Actions on Google