There are some of each. There are unread psych and neuropsych books sitting together on a shelf in my home office next to ones I have read. Then there is this vast land mass next to my bed which grows upward as it spreads out. Those are all unread books which are loosely grouped into categories - fiction is separated from nonfiction, science from other nonfiction, there are a few books on Russian history together, a few books on genetics. The structure is also somewhat archeological - books are naturally organized by when they were received. Those books received more recently are nearer the top and those books I have had for a while but haven't had time for or have lost interest in get buried. Then as something rekindles my interest they rise to the top. For instance, Cornflowerbooks bookclub recently chose Iris Murdoch's The Nice and the Good. I have had a copy of that book for a few years, in fact I have started it twice but never got very far. When she announced it as the next bookclub read, it got promoted to the top of the pile and some rearranging took place so that the whole structure wouldn't come crashing down. There is also a small pile on the top of one of my shelves that is proposed rereads - The Magic Mountain, Jude the Obscure, and His Dark Materials trilogy sit there now. And there is that little tower, artfully arranged, on one of the side tables in the living room that technically is composed of books I haven't read or books I just dip into from time to time - a few volumes of short stories: Mavis Gallant, Edith Templeton, and John McGahern - Semus Heaney's The Redress of Poetry lives there, so does a design book on Parisian apartments, an odd volume of rarely produced Ibsen plays, Amitav Ghosh's The Glass Palace, which I have never been able to finish, Barbara Tuchman's The Proud Tower which I have already read but long ago, and yet another Chekhov biography. There can never be enough books on Chekhov!
Follow-up to last week’s question: Do you keep all your unread books together, like books in a waiting room? Or are they scattered throughout your shelves, mingling like party-goers waiting for the host to come along?