There is a series of books on GIMP.
What are the differences among them? Can you help to pinpoint a good reference ?
Ideally I hope that after reading such a book I would be more or less (depending on the task) able to use all the other main software (Photoshop? Lightroom?), that is I am not looking for a step-by-step guide "click here, open this menu, apply the automatic setting et voilà" with screenshots of the menus but rather for something more enlightening like "to achieve this effect you can reason in terms of layers, so try applying this kind of transformation and then..."