If you have installed Cygwin, you can find lots of documentation in /usr/share/doc/. Some packages have Cygwin specific instructions in a file /usr/share/doc/Cygwin/package_name.README. In addition, many packages ship with standard documentation, which you can find in /usr/share/doc/package_name or by using the man or info tools. (Hint: use cygcheck -l package_name to list what man pages the package includes.) Some older packages still keep their documentation in /usr/doc/ instead of /usr/share/doc/.

There are links to quite a lot of documentation on the main Cygwin project web page, http://cygwin.com/, including this FAQ. Be sure to at least read any 'Release Notes' or 'Readme' or 'read this' links on the main web page, if there are any.

There is a comprehensive Cygwin User's Guide at http://cygwin.com/cygwin-ug-net/cygwin-ug-net.html and an API Reference at http://cygwin.com/cygwin-api/cygwin-api.html.

You can find documentation for the individual GNU tools at http://www.fsf.org/manual/. (You should read GNU manuals from a local mirror, check http://www.fsf.org/server/list-mirrors.html for a list of them.)

Comprehensive information about the Cygwin mailing lists can be found at http://cygwin.com/lists.html.

Comprehensive information about reporting problems with Cygwin can be found at http://cygwin.com/problems.html.