I prefer to write on ruled paper and tuck it into the front cover for the following reasons:
- ruled paper is easier to write on
- you won't run out of space (just add sheets)
- you can rewrite summaries as you revisit books decades later and retuck the new summary
- you can quickly scan the papers to archive a digital copy
- you can remove your notes to lend the book (or not)
- you can transfer your notes to a new edition trivially
- you can be more candid in what you write, knowing it's trivially discarded without giving up the book
- all the notes are in one place and you don't have to thumb through the book looking for where you wrote an idea that you half-remember
- you have space to draw diagrams if appropriate
- you can find something on the internet that importantly relates, print off a page or two, and adjunct your notes
- you can do extensive internet research and build a whole matching folder of contents on your laptop, but you can print a page of Title/URL's to adjunct your notes as a backup (and for anyone who borrows the book+notes)
(edit: formatting)