My Opinions: I like the book a lot better than I like its site. The story is that a grandmother dies and she leaves a sort of treasure hunt for her relatives in her will. This family is apparently the most powerful family on Earth, with a bunch of famous people distantly related to them. The 39 clues should lead someone or some group to a great treasure or something that will make them have a very strong hold over the human race. Something along those lines.
The main characters are young and smart, so they're my kind of people. Amy and Dan Cahill, 14 and 11 respectively. You know, when I read about some of Amy and Dan's competition, Ian and Natalie Kabra, I was somehow reminded of the rivalry between the characters in the Spy Kids series. Remember in the 2nd, Gary and Gertie Giggles? They were who I thought of when I read about Ian and Natalie. So after that I thought of Amy and Dan as Carmen and Junie. I know, this is all probably confusing if you've never seen Spy Kids, let alone read the 39 Clues. You'd have to have done both to appreciate the comparison.
If you don't care for joining the website, that's completely fine. It's not the kind of site where you make friends. It's a lonely search for the clues. You need to play silly games to get them anyway. But if you'll just read the book, you'll find a neat little clue right in it's pages. Hint: Pay special attention to the page numbers, especially near the end.