[Mikhail_Edoshin]

I wouldn't call it easy reading though :) By the way, it's written in Pascal, while all current implementations, afaik, are rewritten in C.

Also, I found "TeXBook" very hard to follow; "The Program" is much more understandable. Every chapter in "TeXBook" starts with a few paragraphs of relatively simply text and then descends into a bunch of additional paragraphs marked with "dangerous turn" signs that talk about things that were never mentioned before. As the author put it, they're explained "somewhere", but it's really hard to find that somewhere.