[tex-eplain] (was optional arguments)

[Karl Berry]

    Would anybody please be so kind to recommend some reading on that

TeX by Topic - http://www.eijkhout.net/tbt/

Although I think Victor's book is the best guide for in-depth hacking,
you might also find TeX for the Impatient worth looking at -
http://www.ctan.org/tex-archive/info/impatient/

There are pdf's and sources available online (under the GFDL) for both
books.  If you want a nicely bound version, they're probably available
second-hand.  Both are long out of print.

Best,
Karl