Meet The Sullivan Detective Agency: Double Trouble — An Adventure-Forward Children’s Mystery Book Series

The Sullivan Detective Agency is a series of mystery books for kids that has attracted a lot of kids already. Those who are unaware of this series can read this blog to learn about all the books that have been published in this series. Think classic stories rebuilt for modern readers: short chapters, breadcrumb clues, and ethical detectives who use brains over bravado. It’s an adventure mystery series that respects kids’ curiosity and rewards it. So, let’s explore!

Why Kids and Parents Love This Children’s Mystery Book Series

• Fair-play puzzles:

  The clues are on the page. Attentive readers can solve the case alongside the twins—very relevant for fans of children’s detective books and classic locked-room setups.

• High-velocity adventure:

  Secret tunnels, old maps, stakeouts, and “impossible” sightings. These are brilliant adventure thriller books scaled for middle grade.

• Heart & teamwork:

  Every win is shared—Carter’s logic, Cooper’s nerve, Kylie’s agility, Brody’s dependability, and Mr. Amos Murphy’s mentorship. It’s a perfect combination of teamwork.

• Reader confidence:

  Short, cinematic chapters make this a strong bridge for developing readers who want the best mystery books for kids that feel “grown up” without the heavy content.

If you’re browsing best mystery books 2025 lists or piecing together mystery books for your kids shelf at home or in class, put these one high of the queue. It fits right beside your family’s thriller and mystery books.

Deep Dive into the Best Mystery and Thriller Books by Parker Kelly

Book 1 — Fear in the Forgotten House

Plot: Whispers about an abandoned Victorian on Willow Street dare the Sullivan twins—Carter and Cooper—and their friends Kylie and Brody to look closer. Strange lights blink in the attic, something scrapes behind a bookshelf, and rumors circle a vanished owner, Mr. Dunn. The kids officially form the Sullivan Detective Agency, fund up with small neighborhood cases, and return with better gear. Inside, they find hints of hidden corridors, recorded whispers, and a layout that doesn’t add up. When one of their own goes missing during a stormy stakeout, the search pushes them deep beneath the house and into the heart of its puzzle.
Why kids will love it: Classic haunted-house vibes, a genuine locked-room feel, friendships under pressure, and that “aha!” reveal that makes children’s mystery book series so satisfying.

Book 2 — The Castle Quest

Plot: At Mr. Murphy’s ancestral castle, the twins team up with local siblings Ciara and Finley. A wolf-like púca is said to haunt the grounds; a dungeon door locks and unlocks by itself; misty shapes drift across their camera feeds. An old map reveals secret servants’ passages and an overgrown entrance by the lake. Boot prints—and convincing paw prints—pull the team into the woods. With tensions rising in the village and odd behavior from neighbors, the kids rig alarms, study the castle’s history, and race to learn whether folklore is hiding a prank, a crime, or something stranger.
Why kids will love it: Irish lore, hidden rooms, and a Scooby-Doo-adjacent unmasking—pure comfort for readers who crave mystery book series for kids with real-world stakes.

Book 3 — The Final Clue

Plot: Years after a boy disappeared at Bloomfield’s shuttered amusement park, the twins explore a “powerless” arcade that mysteriously hums to life. Back at school, they glimpse a massive crate in the basement that looks suspiciously coffin-shaped. A hidden ventilation shaft yields old clues; nighttime trips reveal a sarcophagus that vanishes, then reappears under a tarp. Cryptic conversations about “insured shipments” raise the stakes. With Amos Murphy and investigator Tom Connor, the team gathers proof, outmanoeuvres shadowy guards, and follows a trail that suggests the town’s quiet halls may be cover for something far bigger.
Why kids will love it: Big-movie set pieces (funhouse maze, school break-in), real history (Hellenistic Egypt), and a satisfying sting operation. This is the installment to hand readers who ask for the best mystery thriller books but still need age-appropriate thrills.

Book 4 — A Journey Back to Ireland

Plot: Word arrives that Dunn is somewhere in Ireland—and a headless rider begins tearing through local legend and into real life. After a terrifying close call, the twins trace hoofprints to cave tunnels that crisscross the hillside and spill into dense forest. A festival crowd offers both cover and danger as the figure appears and vanishes. When an ally goes missing and a black stallion is seen near an abandoned cabin, the team follows the trail toward a final confrontation, unsure whether they’re chasing a criminal in costume, an accomplice, or a mystery that refuses to fit the rules.
Why kids will love it: High-tension rescues, folklore that nips at the edges of reason, and a victory that still leaves one goosebump on your arm. It’s adventure-forward without losing the fair-play core that makes children’s mystery book re-readable.

Book 5 — Vampires in Bloomfield

Plot: When moving trucks unload a coffin-shaped crate into the freshly bought house across from the Sullivans, Carter and Cooper feel their detective senses—and imaginations—ignite. The new tenant, Mr. Dalcais, is “from Transylvania,” keeps odd hours, and a caped figure keeps appearing in the streetlight fog. With Kylie, Brody, and new friend Jaxon, the Sullivan Detective Agency sets a camera grid, maps comings and goings, and—just in case—packs old-school defenses (crosses, wooden stakes, a bottle of holy water). To crack the case, the kids will have to separate myth from motive, tighten their timeline, and decide what counts as real evidence—before the next midnight visit.

Why kids will love it: Capes, coffins, and midnight stakeouts deliver goosebumps without nightmares—surely another exciting mystery book for kids.

Who is This Series Perfect for?

• Curious, puzzle-loving readers who want to call the twist a page before the reveal.
• Families and teachers building a shelf of children’s mystery book series that reward attention, not just speed.
• Young adventure fans who love treasure maps, stakeouts, and close calls—aka the sweet spot for adventure mystery books series.

And yes, grownups, you’ll enjoy the craft. The plots nod to the architecture found in the best mystery thriller novels of all time, shrinking the danger to kid-safe stakes while keeping the logic tight. If your book lists lean toward the best mystery and thrillers, this is a terrific read-along option.

What Mystery-Thriller Authors Can Learn from Parker Kelly Books

1. Fair-Play Clues, Earned Twists

Plant every crucial clue in plain sight; disguise it with context, not concealment. Payoffs feel surprising yet inevitable.

2. Setting as the Puzzle Engine

Let place drive plot. Haunted house, castle, amusement park—terrain dictates traps, timelines, and chase geometry. Sketch maps; make space, create stakes.

3. Rhythm That Hooks

Use short chapters and micro-cliffhangers. Alternate quiet deduction with kinetic set pieces to keep momentum without whiplash.

4. Ensemble Detectives, Clean Exposition

Give each detective a distinct competence and solve via collaboration. Surface backstory through action and clue-handling, not speeches. This could be crucial when you aim to write a mystery books series for kids.

5. Series Spine, Standalone Payoff

Thread a recurring antagonist and seed the next case in the epilogue, but close the core mystery every book.

6. Takeaway

Study these moves and you’ll blueprint tightly plotted, reader-involving thrillers that genuinely earn their twists.

About Parker Kelly

Parker Kelly makes her home in Texas with her husband, their children, grandchildren, and three energetic herding dogs. She first picked up a pen in second grade and never looked back, eventually earning degrees in English literature and creative writing from St. Mary’s University. Alongside award-winning titles like There’s No Place Like Home and her Sullivan Detective Agency: Double Trouble series, she spent over twenty years with a major airline—coordinating group travel, training staff, and exploring IT solutions. Trained in EMS at the University of Texas Health Science Center, Parker now runs GBFIT, a strength-and-conditioning studio, and pursues ocean kayaking, hiking, snow skiing, and martial arts.