We are trying to improve the libcoap documentation but this will take time. Meanwhile, you can contribute if you like.
A short quick start guide to get you up to speed fast.
The doxygen-generated documentation for various versions of the application programming interface:
Minimal Code Example
The GitHub project libcoap-minimal provides a minimal client and server example in C++.
Updating Application Code from 4.2.1 to 4.3.0
With libcoap 4.3.0, the public API has been changed considerably. Applications that have been developed for API version 2 will fail to compile with API version 3. The necessary updates to API v2 applications are documented in this Wiki page.