The Red Blazer Girls, by Michael D. Beil, was both clean (although some reviewers mentioned bad language at the beginning, it must have slipped right by me) and engaging. Here is an abridged version of the blurb on the back cover of the book:
"Three unexpected sleuths-in-school-uniforms come to the aid of a strange old lady, and find themselves tackling a scavenger hunt set up decades ago, to search for a legendary ring reputed to grant wishes.... [They are] just three mostly regular girls (like you?), who are surprised to find themselves hiding under tables, trying to solve puzzles and equations, and prowling through moldy storage rooms. Anything else you should know about? Oh, yeah. There's a boy involved. Who complicates things. As boys often do.
"This very first Red Blazer Girls adventure offers a fun, twisty whodunit?, where-is-it? for those who love mysteries, math (c'mon, admit it!), and more than a modest measure of mayhem."
Gotta love the alliteration in the final enticing line! I'm also thrilled to know that my library already owns another episode of this series (although I suspect the puzzles would be easier to solve if you approached them in a hard-copy book instead of in audio). I will be on the look-out for more.