I just read a passionate and convincing argument here for why one should write in books, especially library books.

Back in the way back, books were considered valuable and precious. They were written by experts - who are we to contribute our scribbles, underlining, or highlighting to their wisdom? Books could be valuable, could be resold -- but marks of any sort would lower their value.

As children we were told sternly never to write in library books, so we might have then adopted your process of writing up thoughts elsewhere.

Writing in books can be distracting and misleading.

Even so -- I think I should highlight more, especially the phrases that jump out at me that I might want to find again. Writing in books is have a conversation with the author. I suspect the author would appreciate it.

Sidenote: Much has been made of William Blake's library and books where he left marginalia (by "much" I mean among Blake scholars or biographers).

Be kind to your biographers, flagamuffin, and leave them some scribbles in The Silmarillion