I regged TheDietCode.com There is a book named the same thing. It is not trademarked, but can I still use it legally for a diet affiliate site? Thanks for your answers.
I would say you'd be treading on egg shells. The author of the book could reasonably argue that you are trying to profit from his copyright, obtaining customers through deceptive use of the title of his book. If the product you were offering was sufficiently unrelated to the content of his book you wouldn't have a problem, but as they are both selling diet concepts it might be iffy. Only a lawyer is really going to be able to give you solid advice on it though and even then it might be a case of doing it and seeing if they notice/care enough to serve a cease and decist. Also, the concept behind that book sounds hilariously bad. If you're running a legitimate diet site with sane concepts, I'm not sure you'd necessarily benefit from people mistaking your site for being related to that book
You are wrong. There is no copyright issue with the book title. Copyright Protection is Not Available for Names, Titles, or Short Phrases
You can't copyright a book name. You can trademark a slogan, a phrase, a word. However... Whoever wrote this book and didn't buy the domain name is really uninformed about branding.
That trademark is only for that country but domain is international so his country rules are not for domain. Register it and enjoy