ALA Booklist (Wed May 01 00:00:00 CDT 2013)

The Unseen World of Poppy Malone series, first introduced with A Gaggle of Goblins (2011), continues as 10-year-old Poppy, her siblings, and her professional UFO-hunting parents explore a watery realm. Poppy's levelheaded approach to scientific matters is put to the test when she encounters a group of mermaids. Although this mythical creature is currently popular among writers of all abilities, Harper invests just the right balance of whimsy and skepticism to engage both pro- and anti-mermaid readers. Adding further appeal is the balance between Poppy's exasperation with her family and all she learns from them.

Horn Book (Thu Aug 01 00:00:00 CDT 2013)

In her third romp, Poppy, the daughter of paranormal investigators, is still incredulous of her parents' beliefs, despite her previous experiences. Poppy befriends stubborn mermaid-turned-human Nerissa and then must save her own sister, who's made the opposite transformation. Young fantasy fans transitioning into intermediate novels will find this to be a pleasing, accessible story, but the character interactions are rather flimsy.

Kirkus Reviews

A skeptic trapped in a family of believers comes face to face with nasty nautical ladies in the third book of a series. Poppy's parents may be hokey paranormal investigators, her older teen sister a twit and her little brother an incipient psychopath, but at least she can rely on her twin brother, Will, and their buddy Henry. Yet ever since her family borrowed a houseboat on a lake reputed to be an alien hotspot, she's found the two boys prefer to do things without her. Alone and miffed, Poppy stumbles on the titular mischief of mermaids, and one of them takes her curiosity about the human world to the next level. Unfortunately this results in Poppy's sister Franny's mistakenly sprouting a tail of her own. Can she be turned back before she forgets who she was? Stock characters and sitcom-style dialogue do little to set this series apart from the pack. Still, the action comes steadily, and readers will not grow bored at any point. Oddly, interest in the supernatural seems split almost equally between the Malones' misguided alien hunt and Poppy's very real mermaids. And while the mermaid storyline comes to a satisfactory close, Poppy's falling out with Will and Henry is forgotten entirely. A serviceable fantasy best suited for those kids who have already enjoyed Poppy's previous adventures. (Fantasy. 8-12)

School Library Journal (Sun Sep 01 00:00:00 CDT 2013)

Gr 3-6 Nine-year-old Poppy Malone, whose parents are paranormal investigators, begins to wonder about mermaids when she notices inexplicable giggles and mysterious lights on the shore of the lake at night. She's not sure what to think until Nerissa, a young mermaid, comes ashore and befriends her. While the distracted Mr. and Mrs. Malone work on tracking aliens, Poppy's sister, Franny, accidentally swaps her legs for a tail by touching Nerissa's special cloak. Nerissa's mermaid friends try to lure Franny into remaining a mermaid forever, and it seems like she would love nothing more. Poppy, her twin brother, and Nerissa have a very limited time to attempt to convince Franny to ditch the tail and reclaim her legs. Harper's skill as a storyteller shines through in the solid characters and engaging plot. Substantial yet quick, Poppy's latest adventure will find an audience of eager readers. Amanda Struckmeyer, Middleton Public Library, Madison, WI