Study Stickies allows you to better understand and remember books (both printed and online), videos, podcasts, online documents, etc.

In particular, you can use the service to add notes to your textbooks without using (real) stickies or marking them up in any way. This allows you to more easily browse and search through your notes in one place while keeping your books in good condition.

You can, if you like, share your notes with others. Sharing of notes allows users of the service to learn from each other. For example, if you get stuck on some topic in your textbook, you can see if anyone has made any insightful notes about it.

Check it out: Study Stickies