The only books I automatically buy/preorder are the Terry Pratchett Discworld books. Otherwise I decide on a book by book basis, which is also affected by what I can easily get at the library - my local library is good at getting new releases in quickly and putting several copies on one week loan. This means that I can usually get hold of very popular books quickly for free, and of course I don't have to store them. I can always buy them later in paperback (or now ebook) when they're cheaper if I think I@m going to want to keep my own copy.
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