I only buy books that are not available to borrow from the library. Unless it’s some very special book that I want as a reference or to write notes in, then I can justify buying a book. My library also has the option of suggesting books for the library to purchase. So far I’ve done this once and they actually purchased it within a week or so. I know that doesn’t solve your desire to write notes in a book, but you could possibly keep a journal and make references to page numbers and edition/publication info.