Monsters from a "Pokémon Go"-type game escape into the real world. Bex, 12, is spending the summer before middle school catching virtual hybrid monsters in her favorite augmented-reality game/cellphone app, "Monsters Unleashed," with nerdy best friend Charlie. But after they encounter a strange machine in Charlie's grandfather's attic, Bex's Monster Lab is emptied. They realize it's more than a glitch when one of the monsters attacks them on their way home. Only "Monsters Unleashed" players can see the escaped monsters (though the creatures interact with and affect the real world), and Bex and Charlie figure out that the phone game can be used to recapture the 10 missing monsters. The plot—can the monsters be trapped back in the game before they hurt anyone?—isn't big on surprises, but it is big on action, and the chimera monsters are inventive. Stock side characters include the former best friend-turned-mean girl (and secret gamer) and the bully older brother; their subplots are as subtle as their characterizations. Although Bex and Charlie are white, their Massachusetts town is populated by characters with names that indicate diversity. Post-resolution, the company behind "Monsters Unleashed" releases a new game, which will inevitably cause trouble in the sequel, as will the distance starting the school year seems to place between Bex and Charlie. While there are both better monster and better tech books out there, the fast pace and action focus will appeal to game-addicted readers. (Science fiction. 8-12)