Style guide about standard notation from Indiana University
Some example of non-standard approaches to notation
A pdf about the use of extended notation by some prominent 20th century composers.
Stockhausen used a very detailed notation to specify the sound creation for his piece Mikrophonie 1
Mark Applebaum has been developing sound sculptures while performing them and composing for them. You can read about his work, listen to some excerpts and look at some of his meticulous scores here.
Caitlin Rawley's blog post about graphic and text scores talks about why not just how.
How to typeset proportional notation within the strictures of common notation software.
Anthology of text scores from Pauline Oliveros.
Cardew's manifesto (of sorts).