Ah, the magic of X. I've done my first steps into X by programming a graphical FTP client on a SGI Indy 2 under IRIX. It was 1994 and I spent about 3 weeks and I had luck to get the O'Reilly X books [0] ( and other two, can't remember ).

[0] http://shop.oreilly.com/product/9781565920156.do