To download Open Music : http://forumnet.ircam.fr/shop/fr/forumnet/43-openmusic.html
Documentation : http://support.ircam.fr/docs/om/om6-manual/co/OM-Documentation.html
To download OMChroma : http://forumnet.ircam.fr/shop/fr/forumnet/57-libraries-openmusic.html
To download Csound : http://www.csounds.com/downloads
To download Csound 5.x : http://sourceforge.net/projects/csound/files/csound5/
There is a very large documentation on csound (see http://www.csounds.com/docs). This includes several catalogs of instruments, as well as a few books (see http://www.csounds.com/cdroms/).