Tutorials/manuals for writing class and style files in LaTeX? [closed]

Question

I've searched the net and stackoverflow and found a number of great LaTeX sources, but I couldn't find any decent manual about writing your own class and style files. Some issues I have is which part of the code should be in a class file and which in a style file, how to finetune macros, how to define and use variables,...

I figured some things out by looking at other peoples code, but I'd like to have some comprehensive overview. Any ideas where I should look?

PS : I have already the not so short introduction to latex, introduction to beamer and latex tutorial - a primer. None of these answers my questions.

Answer 1

Sorry, found related questions and answers here and here. Courtesy Matt for the pointer.