Which is useful, pointing out as it did some gaps. I was sure I had a copy of Midnight's Children, but it isn't there. Likewise Tristram Shandy and Marx's Capital. Which last made me realise I'm also missing copies of books which were life changing by Trotsky, Gramsci, Luxemburg.
I've also weeded out for onward transmission to a charity shop on Duke Street several volumes which I've been meaning to read and decided I never will: The God Delusion, for example. Books that I will read, but haven't yet, have a shelf of their own in the sitting room.
Once read, the decision to keep is based on whether I'd really want to re-read, lend-out, or refer to them. Otherwise, it's Duke St. Which kind of tells you which books I regard as "literature", maybe. Le Carre has made the cut, for example, but Iain M Banks' Culture novels, (I had all of them) went to the Salvation Army when we were packing.
Another way of looking at books-to-keep, is that you can look at the spine of it and see an old mate. Whereas the Culture novels are a bit like an old mate you're slightly embarrassed about.