The catch is that if you don't buy a book because it has drm, then the publisher will think that it sells badly because the story is crap... This can certainly happen to new books, or newer authors.
And GOG.com.... They are "zee zjit" as the Germans would say
They prove that you can make loads of money selling stuff without drm, at low prices, even if it's very old. If I want to suddenly play an old game (I have a very rare affliction, called "Bouts of Nostalgia"), then I'll just buy it at Gog.com for $3 or $5. As their games are fully patched up, often adjusted to run under Windows7 and include lots of extras, I won't even start looking for pirated versions.
I actually bought some games I have on CD already, to easily get a good quality mp3 sound track... And have a fully patched, "no CD required"-version to boot.
No drm. No hassle. Good user experience. No restrictions. Nice extra's. Low prices.
That will cause you to sell for millions. Gog.com proves it.