Where can I get a good article about understanding X? I''m currently familiar with the whole client/server concept in general but would like to know more. Particularly with regards to stuff like font servers, xdm, security etc. Does anyone know of such books/articles?

Have you tried
http://www.xfree86.org
http://www.x.org/

For security, start with man xhost.

XDM, GDM and KDM will pop up results in Google. =)

Int.

An old and good book http://www.ac3.edu.au/SGI_Developer/books/XLib_PG/sgi_html/