Books
Books contains the notes mostly extracted from many classical textbooks. These notes are just grouped according to the book title:
Nowadays ebooks are more convenient and available than traditional paper-based books. However, you cannot flip through an ebook as easily as in a paper-based book to locate the content you want to recap. So if you are also an ebook-fan like me, I believe it is a beneficial habit to turn your reading scratches to systematic notes like above.