Maybe important to know (if you run across XLisp for the first time) is that there exist several different XLisp versions, which are unfortunately more or less incompatible to each other. The "X" in XLisp stands for "experimental" and that's what XLisp is most about.
XLisp 1.x - I have never worked with this version and cannot tell much about it.
XLisp 2.x - is an object-oriented Lisp and one of the predecessors of Common Lisp, but instead of CLOS, XLisp_2.0 uses a SmallTalk-like message passing object model. Shortly after the release of XLisp_2.0 by David Betz (original author of XLisp) in the early 1980s, the Common Lisp discussions began.
XLisp_2.0 then was further developed by a group of programmers around by Tom Almy, who tried to make XLisp_2.0 more compatible with Common Lisp. They released several XLisp_2.0 derivates under Names like "XLisp_2.1" and "XLisp-Plus", which are only partially backwards compatible with XLisp_2.0.
XLisp 3.x - is a complete rewrite of XLisp_2.0 to Scheme, again by David Betz, but more or less fully backwards incompatible to XLisp_2.x.
The official XLisp Homepage of David Betz (original author of XLisp) can be found here:
XLisp 3.0 homepage:
http://www.xlisp.org/
I myself have worked several years with XLisp as part of the "Nyquist" programming language for sound synthesis and music composition by Roger Dannenberg, which is based on XLisp_2.0 by David Betz:
Nyquist homepage:
http://www.cs.cmu.edu/~music/music.software.html
Over the years I have collected and HTML-ified all documents and papers I could find in the net about XLisp_2.0. You can read and download the XLisp_2.0 documents collection for free from the german Audacity forum. Audacity is a free audio editor with an integrated Nyquist/XLisp interpreter.
XLisp_2.0 documents collection:
http://www.audacity-forum.de/download/edgar/nyquist/nyquist-doc/xlisp/xlisp-index.htm
Please note that this document collection is not very useful with the current XLisp_3.0 version, which is based on Scheme. The XLisp_3.0 documentation can be downloaded from David Betz's homepage (see above).
For the sake of completeness and because there are still many XLisp_2.1.x (the "x" is a lowercase letter from "a" to "g") and XLisp-Plus versions around in the net, here the link to Tom Almy's XLisp-Plus page:
XLisp Plus homepage:
http://almy.us/xlisp.html
There also exist several other XLisp derivates:
S-Lisp documentation:
http://sig.biostr.washington.edu/share/skandha4/html/slisp_toc.html - XLisp + the skandha4 3d-graphics library
Winterp homepage:
http://nielsmayer.com/winterp/ - XLisp + the Motif graphics toolkit
(X)Lisp-Stat homepage:
http://www.stat.uiowa.edu/~luke/xls/xlsinfo/xlsinfo.html - using XLisp for statistical computations
To answer Paul's "maximum of a list" question it would be very helpful to know the exact XLisp version number and wether it's one of the David Betz original versions or one of the many derivates.