I've just started writing my own PL/0 compiler, I found the best resource for it (apart from the Wikipedia articles on EBNF, PL/0 and P-Code) is Wirth's book "Compiler Construction" (http://www.ethoberon.ethz.ch/WirthPubl/CBEAll.pdf). I don't think later versions of "Algorithms + Data Structures = Programs" contains anything about PL/0 (or a I could not find anything in the contents page). Eitherway, I find Wirth's writing amazingly clear.