This is Google's reference implementation of OGC KML 2.2. It also includes implementations of Google's gx:
extensions used by Google Earth, as well as several utility libraries for working with other formats.
All of our documentation is on the project wiki: http://code.google.com/p/libkml/w/list
The wiki contains documents that describe:
The main project page has links to the discussion group and the mailing list that mirrors the commit and issue logs.
Note on strings in libkml: As of r680, we changed all use of "std::string" to just "string". This change was motivated by a desire to permit use in codebases that provide their own string class. This change was transparent to you because of this change to src/kml/base/util.h:
#ifndef HAS_GLOBAL_STRING
using std::string;
#endif
All string-using code in libkml was modified to pull in that header.
[The above is the old README from libkml.googlecode.com. The wiki pages will be migrated across to github soon(-ish).]