I buy copies of books that I have read, and are worth buying. More likely, I will buy an e-book copy of a book worth buying. E-Books are preferable because I can carry my library with me. But, there are the books with exquisite photos, graphs or diagrams that need the full experience. I lost a large book library in a natural disaster, over a decade ago. I won't get caught that way again.

Edit add: I seldom read fiction. I jam too fascinated of science, architecture, gardening, programming and math. Fiction doesn't reach me the same way. I currently am stuck in a morass of predictable-plot fiction that feels formulaic, and have not found a book that is worth reading for quite a while.