I found this question on this Swedish book blog.
How important is it for you to own your books?
Most of the time it's very important. As long as I like the book. That's why I usually borrow e-books at the library to 'try before I buy'. If that's not possible, I read the blurb very carefully and if possible I read the first page or so on the bookstore's site. Fortunately it's usually enough to be able to tell if I like the book enough to buy it.
Do you buy all books you read or do you prefer to borrow?
As I mentioned above, I use the library a lot, but if I like the book, I'll buy it. Usually, though, I tend to download free legal books, and even among them I've found some gems that I have bought in print later.
When it comes to books you want to own, is it important for you to have all the books in one series and in the same format (paperback/hardcover, same edition etc)?
Yes, if possible. Ursula K LeGuin's books about Earthsea is one example of an exception to this rule. The same goes for Janet Evanovich's Stephanie Plum books. When it comes to the former, to begin with I borrowed the first books from the library (in Swedish), but book 1 wasn't available. I ended up buying the first three books in paperback, in English. Then all the other books in the series were published, or at least I found out about them. So now I have book 4 in hardcover, in Swedish, but all the rest of the books in paperback, in English.
I think found the first book about Stephanie Plum I found on sale in an ordinary supermarket, in Swedish, at least that's how I remember it. I got the other books in English.
I even had to buy an e-book once, though I rarely do so, because book 3 (Hespira) in the series about Henghis Hapthorn by Matthew Hughes, wasn't available in print, except in a hideously expensive hardcover edition. I'm not exaggerating. Apparently, there was some trouble with the publisher. Then the book was published in paperback so I bought it right away.