The NEXUS Class Library is a C++ library for parsing NEXUS files. Paul Lewis originally wrote the library and did a very nice job of documenting its interface. That is the version 2.0 that is documented at http://hydrodictyon.eeb.uconn.edu/ncl/.
Mark Holder has extended NCL and currently does most of the maintenance work. Version 2.1 strives to be backward compatible with v2.0. The documentation for v2.1-specific features is not as good as v2.0 docs, but we are working on them. They improved a great deal on March 27, 2010. Those docs are posted at http://phylo.bio.ku.edu/ncldocs/v2.1/funcdocs/index.html. They refer to some functions that were just added, so you'll need the latest version of NCL.
We are trying out posting Debian packages for NCL at http://phylo.bio.ku.edu/ncl.