Other sources of information about Overpass API.
Language Reference
Systematic and exhaustive enumeration of all elements of the query language
including their formal definition.
Installation of an Own Instance
How can I set up my own instance of the Overpass API
to be able to execute an arbitrary number of requests?
Component Overview
This page gives an overview
which files exist in Overpass related directories,
which processes run during executing an Overpass API instance,
and how the parts work together.
Wiki
Page of Overpass API in the openstreetmap wiki.
Contains for example the list of public instances including their status messages.
Also contains examples for queries
and further examples for advanced usage.
Mailing List
Questions can be asked here.
Outages and bugs can be reported here as well.
Roland's Blog
Special subjects are covered in more depth there.
Also the features of new versions are presented there.
Source Code
Source code of Overpass API.
Also available are Overpass Turbo and this manual.