Managing Your Digital Stuff - Books

Wikipedia: E-book 
  An electronic book, also known as an e-book or eBook, is a
  book publication made available in digital form, consisting
  of text, images, or both, readable on the flat-panel display
  of computers or other electronic devices. Although sometimes
  defined as "an electronic version of a printed book", some
  e-books exist without a printed equivalent. E-books can be
  read on dedicated e-reader devices, but also on any computer
  device that features a controllable viewing screen,
  including desktop computers, laptops, tablets and
  smartphones. The advantages of E-Books are many.
Wikipedia: Audiobook   

  An audiobook (or a talking book) is a recording of a book or
  other work being read out loud. A reading of the complete
  text is described as "unabridged", while readings of a
  shorter version are an abridgement.

You don't have to keep all the E-books and Audiobooks you have purchased (and downloaded) on your devices. You can delete the ones you are not currently reading (or listening to) and re-download later as necessary. Good houseking practice--get it off of your devices when finished reading or listening.