title: What's the best way to understand the TeX / LaTeX internals?
body: <p>Looking through <a href="http://tug.org/interest.html" rel="noreferrer">docs on the TeX website</a>, it seems like almost all documentation for TeX and LaTeX are tutorials on how to <em>use</em> them, how to install fonts, how to get graphics working, etc.</p>
  
  <p>What should I look at if I want to know more about <em>how TeX works</em>?  For instance, I want to know about its parser grammar.  I want to know how its text engine works, how it decides hyphenation, how it flows text, how it decides on box layouts.</p>
  
  <p>It seems like TeX and LaTeX are built up on several layers of different technologies.  (Metafont, etc.)  Is there any resources that describe step by step the TeX system from the ground up?  How it goes from parsing a text file, into how this is represented and transformed, and finally how it ends up in a particular page layout?</p>