I haven't seen any tutorials for writing an emacs-like editor, but I think studying the source of QEmacs [0] and mg [1] is a good starting point.

[0]: http://bellard.org/qemacs/

[1]: http://homepage.boetes.org/software/mg/

Also, you could take a look at these:

http://www.emacswiki.org/emacs/EmacsImplementations

https://wiki.gentoo.org/wiki/Project:Emacs/Emacs-like_editor...

http://texteditors.org/cgi-bin/wiki.pl?EmacsFamily