How do you know a book or website is valid?

There are standard protocols for most forms of publication. Book have been around longer so these protocols are well-developed. The include the book providing details of author, publisher, date and subject matter. There are also several forms of reference numbers. Or just read the book objectively and decide for yourself.

Websites will have a URL which normally contain a domain name. A good website will have details of the publisher such as their location and how to contact them as well as a privacy policy. They will often use a thrird party payment system such as PayPal.

Again, however, you have to use your own judgment when looking at content.