As little as I can get away with. I read too much (probably around 100 books/year) to be able to accumulate the physical volume and electronic formats are just

As little as I can get away with. I read too much (probably around 100 books/year) to be able to accumulate the physical volume and electronic formats are just much more convenient. Usually, it's long tail (specialist), older books. Sometimes, surprising ones: Lee Kuan Yew's arguably most famous book, "From Third World to First" is only available in paperback or hardback (some of his later writings are available for Kindle).