All things being equal (money, space, etc), would you rather own copies of the books you read? Or borrow them?
Considering all the things are not equal, I borrow most of the books I read. I buy only the books I really love and the books which I know I will keep rereading. Sometimes, it happens that I first read a borrowed book and then I love it so much that I go and buy it.
However, if money and space were not the issue, I would be buying much more books I do now. I enjoy to be surrounded by the books I love.
It is quite a coincidence, this week's question comes right as I am on the lookout for some additional bookshelves (which would suffice for next few years, I think).