Not sure about the license. Looks like it’s not specified. But on their “About” page they tell you that you can get in touch with them to contribute. They also list the contributors.

"Please contact us if there are errors to report, things that you think should be improved, or if you have ideas for better exercises etc. We sincerely look forward to hearing from you, and we will continuously improve this book, and add contributing people to the acknowledgement.

Contact us here!"

Anti-features include them disabling right click. On Firefox you can permanently ignore such thing by going to about:config and setting dom.event.contextmenu.enabled to false