Book Scavenger by Jennifer Chambliss Bertman
I’m only ~50 pages in but I’m liking it so far. The pieces are being set down so I can see how it’s going to be a fun mystery. It’s got a nice MG voice going on. It’s not too juvenile or off-putting (so far). I’m a little bummed that Book Scavenger isn’t a real game (but you can hide and find books on the book’s official site).