While viewing a page in a flexbook, you can choose to highlight a section of text or make a stickie note. To do this click on either the highlighter or note icon floating by the right edge of the flexbook page. Now you can click and drag to highlight or leave a note for a section of the text. You can change colors for both items by clicking on the arrow next to the respective icon.
Note: All annotations and highlights will remain on the book for that specific version and are only visible by you. It is best to make a copy of the resource, that way your notes will always be easily accessible to you from your library.