Masteringemacs.org sells a well-known guide to the Emacs text editor, and the site has been online since 2010 with a solid security setup and no history of abuse. That's the good news. The concern is transparency: there is no email, phone, or address listed anywhere, and the homepage uses a "flash sale" countdown to push a purchase. For a legitimate educational resource that charges real money, that's an unusual way to operate. The domain and SSL certificate both expire in about a month, which could mean the owner is less attentive than you would want for a paid product.
If you are considering buying the Emacs 29 Edition, look for independent reviews of the content itself (the book has a good reputation in Emacs circles) and check whether the author provides a way to reach them outside the site. Masteringemacs.org is not obviously a scam, but the missing contact info and aggressive urgency tactics mean you should treat it with the same caution you would any other online storefront that makes it hard to get in touch. Consider using a payment method with buyer protection just in case.