My DS7 loves mysteries, so I'm constantly on the look out for good ones. His favorites so far:

Encyclopedia Brown- good, short mysteries that let the reader try to solve it before they reveal the answer and how Encyclopedia solved it. Grade level: mid elementary for difficulty, but DS was devouring them at 6.

Series of Unfortunate Events- he started this series recently and I worried that it was too dark, especially because it's a mystery without a happy ending. It actually ended up being a good bridge between the lighter, fluffier mysteries written for 3rd or 4th graders and the somewhat darker books written for Middle Schoolers. Grade Level: mid-late Elementary, but not for really sensitive kids.

Great Illustrated Classics- there are 3 Sherlock Holmes Mysteries available from them. They are very, very abridged versions of the originals, with illustrations that take up whole pages and huge typeface. But my DS loved them! The books inspired him to look for the original Sherlock Holmes books and while he wasn't able to read them yet, I am all for a book that motivates kids to find out more, even if it isn't the truest rendering of the original stories. Grade Level: difficulty is probably mid elementary, but the text size and copious illustrations make it accessible to much younger taglets.

Nancy Drew and Hardy Boys, the original series- DS found these a few weeks ago and is absolutely in love. I like that they're written at a higher level, but because they're from the 50s, other than some mild sexism, there aren't hugely inappropriate themes or situations. It's also nice that both series have a huge number of books, which can be found for dirt cheap at garage sales and Goodwill when we exhaust our library's offerings. Grade Level: Middle School for difficulty, but the text size and occasional illustrations make the books friendly to 6 or 7yo taglets.