- 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)
The levels of waste produced in the modern era are unimaginably higher than then, as well, so our responsibilities are completely different.
Post-its work. For me I use a mechanical pencil.
A book you won't write in is a book that owns you, rather than you owning the book. A book is designed to convey information on the page, writing in your book only adds to and fulfills it's purpose.
I mean, suppose you have a 100 books. You're not reading more than 2 or 3 of them at once, if that; and it's not like you're doing anything with the rest other than keeping them on a shelf.
Now, Once you either have a family, or have friends with similar interests, or with kinds of similar interests etc - those 98 (or 100) books become something you lend, or even give away. And when you get older, you will pass on your entire library, at some point, or sell it off, or bequeath it etc. At any of these points, you want the book to be free of your personal jots and notes, and as close to pristine condition as possible, for the other readers.
Write your notes in a notebook, or on your PC or laptop (or whatever). They'll also be searchable and easily editable.
While I generally have a communitarian orientation, I predict that as I approach this point, other end-of-life matters will occupy my headspace to a greater degree than any psychic rumblings about the odd marginalia in my dusty collection.
As an aside (because for the mere mortals among us it doesn’t apply) occasionally some historical inferences are made based on the reader’s margin notations.
https://siarchives.si.edu/blog/post-it-or-not-post-it
http://libraries.ucsd.edu/preservation/postits.html
It's not clear from these articles how soon damage becomes apparent; I haven't noticed any yet in books which had Post-Its. It may not matter if you don't intend your books to have archival value.
I've tried Book Darts for marking pages, and they can also be used like paper clips to hold notes. I write short notes in pencil, but try to avoid dark leads which make marks hard to erase.
I have at least a few Post-it notes in every book. Some sitting there for thirty years or more. Never experienced any problem. Always used the real Post-it from 3M.
https://web.archive.org/web/20110128174227/http://libraries....
I do keep a notebook where I write about random stuff and a lot of thinking about what I’m reading goes in there.