The main references are:
Fischbacher, Urs, Katharine Bendrick, and Stefan Schmid, 2019. "z-Tree 4.1 Tutorial and Reference Manual." Available on-line at: https://www.ztree.uzh.ch/static/doc/manual_v4.pdf
Fischbacher, Urs. 2007. "z-Tree: Zurich Toolbox for Ready-made Economic Experiments", Experimental Economics 10(2), 171-178.
z-Tree home page: the official z-Tree web page.
Ernesto Reuben's page: contains some useful examples, and a openzleafs.exe, a handy file to launch multiple instances of z-Leaf.
There exists also a mailing list for z-Tree users. To join this list refer to one of the following instructions:
z-Tree unleashed: z-Tree can now also be run over the internet.