Sissy - that is exactly the problem! We have lost sight of what real food is! If you are struggling to work out whether a food is real, natural food, think about whether it would have been available (in that form) to your great, great grandmother.
And no, mash with butter is not a bad food. Sure, if you ate huge piles of it every day it would not be ideal, but that is the point. A proper balanced diet should include fatty things, some starchy things, meat etc balanced by plenty of veg. There was not an obesity crisis in our great great grandparents' time. And less cancer, diabetes and heart disease. And yet they were eating butter, lard, loads of stuff that we have been told is bad.
People then would have died of other stuff of course, but we are lucky enough to have modern medical treatment. It's a shame we are wasting our potential for better health and healthier long life by stuffing ourselves with such rubbish.
Goodness - apologies for sounding so evangelical! This is all very new to me too really (although my recent phase of low carb and paleo had already made me cut out most of the crap), and it's been quite a while since I believed in the low fat diet.
For me, the difference this particular book has made is the feeling of letting go of all that diet-baggage and choosing real foods because I like and enjoy them. The pleasure of thinking 'Mmm - what shall we eat today?' Instead of working out how to make a meal containing nothing too 'bad'.