I don’t really – not really-really – know anything until I’ve copied it out, by hand, with pen and paper. Note-taking helps me to memorise the most useful, interesting, beautiful or aggravating parts of a book. It also means that whenever I want to retrieve a reference from something I’ve read, I can find it in my notebook. Not marked with a torn-up train ticket and then replaced on a bookshelf but I’ve forgotten which bookshelf, or given to a charity shop in the hopeful belief I’d never need to think about it again; but in my notebook, with a page number, marked on the contents page.
Ditum goes on to describe her method and tools. See also this post about keeping a journal and collecting.