DS 6 is on the last of the Mysterious Benedict Society and he loves them. I have read 2 1/2 of the 4 and enjoyed them. He also loved Harry Potter and Geronimo Stilton books. Mr. and Mrs. Bunny Detectives Extraordinaire is also a great book, as is The Girl Who Circumnavigated Fairyland in a Ship of her Own Making. A 6 YO will miss out on some of the linguistic nuances, but the stories are compelling on their own.