The URL for Net::Z3950 is http://www.miketaylor.org.uk/tech/nz/index.html
I'm not going to talk about it :-)
The tutorial can be found at http://www.miketaylor.org.uk/tech/nz/doc/Z3950/Tutorial.html
But these principals apply equally to other languages.
The abstract version of the API is embodied in ZOOM, which can be found at http://www.miketaylor.org.uk/tech/zoom.html