I have just wrote a little tutorial to show how images can be edited in Lab colorspace using the PhotoFlow GIMP plug-in.
The goal is to allow performing at least 90% of the edits that Dan Margulis describes in his books…
The link to the tutorial: http://photoflowblog.blogspot.fr/2016/10/non-destructive-lab-editing-with-gimp.html
Enjoy!