A reading note is most useful when it preserves both an idea and a route back to the page. A memorable sentence without its context can become difficult to evaluate once the book is no longer at hand.
Record the edition and the location of the passage alongside your note. Distinguish a direct quotation from a paraphrase, and keep your own response in a separate paragraph. This simple division makes later writing easier to check.
Add a sentence explaining why the passage matters to your question. It may offer evidence, a different interpretation or a lead to another source. Naming that role prevents a notebook from becoming an unstructured collection of appealing excerpts.
At the end of a reading session, review the questions that remain. A useful note does not need to summarise the entire book. It needs enough context to support the particular enquiry and enough precision to let you examine the original passage again.