"The 39 Clues" books are sort of the Monkees of the kid-lit book world. It's not a natural construct, but something put together by a publishing house--Scholastic--to make money. The idea is this: pick one author to write a general story arc for a series of 10 books, then play a sort of telephone game with the writing of them--each book gets written by a different author. Each author is allowed to put their own spin on things, within reason, but they have to stick to the main arc and get their characters to the desired place by the end of the book. Basically, with the end of the Harry Potter series, Scholastic had to come up with some new ways to get kids to buy their books. This was one of the things they thought up.
Part of the reason for the series is to tie it all into a website and a series of clue cards--some of which come with the books, some of which are sold separately. The idea behind that is to pull the internet generation into the series and create a web/book synthesis. The books provide about 10 of the 39 clues, and many of the others can be found on the website. I just spent some time on the site, and it has some interesting games and things on it.
As for the books, they are very quick reads and the overall story is interesting. The basic concept is kind of fun: there is one family, the Cahills, who are behind almost everything in history. Members include: Edison, Marie Curie, Benjamin Franklin, Napoleon, Mozart, Amelia Earhardt, Deng Xiao Ping, and just about any other historical figure you could name. The family is divided into four branches: an inventor branch, a artistic branch, a power-hungry branch, and a branch known for physical strength. At some point, the family developed a great and powerful secret. But the secret has been hidden for hundreds of years. In order to discover it, you have to find the 39 clues. Some of the family branches have discovered some of the clues, but you need to bring them all together. If you do, then you have the most powerful secret on the planet.
Sent to hunt for these clues are a number of teams. One from each family, plus the lead characters, Dan and Amy Cahill. The books take Dan, Amy, and their au pair, Nelly, around the world and into many dangers. In the end, though other teams may have gathered a clue or two along the way, Dan and Amy, so far, have been able to collect them all.
There is definitely a difference between the different authors. Some of the books are simply better written than some of the others. The books, however, are very expensive. There will be 10 books in total, and each book is $13.00. That's a lot of money for books which can be read in a couple of hours. These are definitely library books. They're not good enough to go out and buy $130 worth of book, but they'd be good to borrow for a little light reading.