Apart from textbooks, which one always ends up buying regardless of price, the following seem like well-motivated purchases:
- Books to travel with. Three library hardcovers are a lot heavier than three paperbacks. Besides, you can't slough library books.
- Books with passages I return to. Poetry, of course; but also the more aphoristic types of nonfiction. Montaigne or Johnson, say. Some literary criticism -- W.H. Auden's The Dyer's Hand, William Empson's Some Versions of Pastoral. Some picaresque novels, like Tom Jones or Don Quixote, that are readable in sections. Not Dostoevsky or especially Tolstoy; if I were to reread War and Peace, I would probably have to reread the whole thing. It follows that I don't need a copy, since I can borrow it whenever I want to reread it.
- Of course, one's very favorite books are always rereadable in parts.
- Remaindered books. The bargain/remaindered books at most bookshops are, of course, there for a reason -- there are very few masterpieces, but a lot of the lesser works of good writers.
- Books perpetually checked out of the library. Camus, for instance, or Arendt or anyone else of that sort. I tend to avoid these except when I can get them cheap. Which isn't always possible.
Of course, I often make stupid purchases, like buying books I probably won't reread because they're on sale.
Given that one wants a book, there's a decision to be made about whether one should hang on until one finds it at a bookshop or use Amazon. I like to support what Mary Beard calls "Real Bookshops," i.e. independent bookshops with better collections than Borders/B&N. I don't mind the big-box stores, but they tend not to have what you're looking for, and, more importantly, not to have anything you didn't know you were looking for. I don't think these bookshops serve a useful purpose, except as pretentious cafes; I would rather support Amazon, which tends to have better prices and more books in the long tail, where most of my reading list is. I especially like shops with medium-sized, idiosyncratic collections; there's something to be said for the acres-of-random-stuff model, but one tends to zone out and miss things one isn't specifically looking for.
Apart from the social responsibility high, I guess there's the rationing effect of saying something like, that's a book I want to own, but I'll wait until I find it at a bookshop.
[These reflections were triggered by seeing the name Alcock somewhere in a news story, and remembering this observation in Littlewood's Miscellany p. 128:
At a Scholarship Examination, Dykes pointed out to me that the list had the consecutives Alchin and Alcock.]