Can I use the phrase "law of attraction" in a book I am writing and will promote on the internet?

Yes, you can use that phrase in the title and body of your book. Short phrases like that one aren't protectible by copyright. There are no doubt dozens of books, films, songs, etc. with that phrase as their title and countless more that include them elsewhere. Bret Easton Ellis has a novel called, I think, "Rules of Attraction."

If you have other questions about rights you may be violating in your book and intended marketing of the book, you should run it all by an IP lawyer to vet to help make sure about those rights, including privacy, publicity, trademark, copyright, etc.

Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.