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)