Harbour compiler is compatible with CA-Clipper, so, any Clipper manual could be useful to use as reference.

A complete Clipper reference can be found at the following address:

http://www.itlnet.net/programming/program/Reference/c53g01c/menu.html

Harbour team had included various extensions to Clipper language, such as class creation, support for date constants, FOR EACH and SWITCH structures, etc, but these extensions are not documented yet.

All aspects refered to Windows GUI are managed by HMG library (the heart of HMG). Documentation is included in HMG package. This available online too: http://www.hmgforum.com/hmgdoc/hmgdoc.htm