All of the methods mentioned - writing on a piece of paper, highlighting key points, underlining supporting ideas, and putting questions in the margin - can be used for taking notes while reading a book from the library.
However, the most effective method may depend on personal preference and the purpose of the notes. Some people prefer to write notes on paper, while others may prefer to use digital note-taking tools. Additionally, the purpose of the notes may also influence the preferred method - for example, if the goal is to summarize the main points of the book, highlighting and underlining key ideas may be more effective, while if the goal is to engage critically with the text, writing questions in the margin may be more useful. Ultimately, the most important thing is to find a note-taking method that works well for you and helps you achieve your goals.