I have a few ideas:

The Last Treasure by Janet Anderson

Running Out of Time by Margaret Peterson Haddix (a fav of my DD)

The Finnegan Zwake series by Michael Dahl

Search for the Shadowman by Joan Lowery Nixon

The Westing Game - Ellen Raskin

The Kidnapping of Christina Lattimore - Joan Lowery NIxon

Here are some that are on my shelf: (that my 13 year old enjoyed)

I would be willing to do 5 for 3 on these or any other of my kids books.

Mysteries in the national park series;

Night of the Blackbear

Buried alive

Mary Kate and Ashley Mysteries my 13 year still loves them

Cat sitter Mystery

The Chameleon wore chartreuse a chet Gecko Mystery

Babysitters Club  Mysteries(several on my shelf)

Ghost of Popcornhill

Ghost writter series

Hit and run Holiday Nancy drew case file number 5

Lizzie Mcguire mysteries case at camp get me outie

Midnight chess game(I  think this is a mystrery but my child really enjoyed it)

Montezuma's missing treasure

Mysteries for midnight

true crimes and how they are solved. This was really interesting. It talks about when they started using fingerprints, dna testing adn the crimes that were solved with them. 

I will be happy to answer any questions about these.  I hope this helps.

Hardy Boys and Nancy Drew Books are good mysteries.  Not sure if they would be a bit young for her or not.

My 12yo daughter loved all the Charlie Bone books.

Chasing Vermeer and Peppermints in the Parlor are 2 good mysterys that are on my bookshelf and I am doing 2 for 1 right now. :)

I love mysteries, and Lois Duncan was my favorite author at that age. 
I do have some mysteries on my bookshelf and a deal listed on my profile if you'd like to check it out.