The best notebook app for iPhone & iPad is Evernote . It is a free app. It is great for taking notes on the fly, or for archiving information you might find online. It is available on the iPhone and iPad, so you can use it for any quick notes or for larger projects. If you have the iPad, you can also use the Apple Pencil, which makes it far easier to digitally draw diagrams. I personally use Evernote every day, both professionally and personally. It's a great way to organize your thoughts and research.