Solved!
@JSWolf: I admit I'm sometimes monomaniacal on small issues, but I bought an ereader for one reason: to
not have a back- or front-lighted device, but something more like a book (there are enough lighted devices at home: TV, computer, phone, iPad!). Setting brightness to 0% was not satisfactory because family would aways put it on again, etc. For me this is a good example of: less is more.
Anyway problem solved
