... treat a DRM-free game like a physical book. ...
I'm afraid that's a misconception (regarding most legal systems). When you buy virtual goods (games, e-books etc.) you generally only buy a personal user licence. Where when you buy a physical copy, you also get ownership of the physical medium. That's why you are allowed to sell, gift or lend the medium (you are the owner), but not a user licences (once it's bound to you) because it doesn't contain actual ownership of the data.
I wasn't talking about laws or licenses (hence my comment about obeying the spirit -- while disregarding the letter -- of the rules). I was simply talking about ethical use of goods for which one might have payed a purchase-level price. For legal reasons, GOG can't officially
condone this type of thing, but several of their staff have mentioned sharing games with friends as being one of the major advantages of DRM-free, and one of the things they missed about the old days. (I think TheEnigmaticT might have even publicly used the "book analogy" as something that he personally thought was OK). I think the "book analogy" is a good guideline...but only if the person to whom one is lending the software is trustworthy.
Edit before posting: ZFR ninja'd me (much more succinctly, too), but I might as well post this anyway. :)