Weinstein Books is not known for publishing children’s books, but that hasn’t stopped the publisher from branching out in the category. This summer the publisher is releasing a new series of chapter books, Secret Agents Jack & Max Stalwart by Elizabeth Singer Hunt. The series, for readers ages six to nine, is a spin-off of Hunt’s debut series, Secret Agent Jack Stalwart, which has become a handselling favorite as well as a go-to choice in schools for reluctant readers.

Despite little media coverage, the 14-book Jack Stalwart series, originally released between 2007 and 2012, has sold roughly 585,000 copies. The series, which Hunt conceived while on safari in Kenya, follows the adventures of a globe-trotting nine-year-old spy. Each volume sees young Jack in a different country, immersing himself in the local culture while preventing evildoers from succeeding in their nefarious plots.

Hunt, an American who lived in the U.K. for eight years after marrying an Englishman, self-published the series more than a decade ago. Subsequently acquired and reissued in England by Random House U.K., it became a bestseller abroad. (Among other highlights, the series was selected by the popular Richard and Judy Children’s Book Club and named a “must read for boys” by the British education secretary.)

Although the series has not sold as well in the U.S. as in the U.K., it has been a hit with two important groups: indie booksellers and librarians, who have helped introduce it to reluctant readers. Weinstein’s publisher, Georgina Levitt, said the indies in particular “have been instrumental in helping us develop great relationships with local schools.” The Jack & Max Stalwart series has been leveled for Common Core state standards, further enhancing its appeal to elementary school teachers in grades one to four and to school librarians.

The new series features Jack traveling the globe alongside his older brother Max (who, handily, is a 12-year-old cryptology expert). The first two volumes in Secret Agents Jack & Max Stalwart, The Battle for the Emerald Buddha and The Adventure in the Amazon, were released on July 25, along with an activity book by Hunt that complements the series, The Secret Agent Training Manual: How to Make and Break Top Secret Messages.

The indie booksellers PW spoke with who had championed Jack Stalwart said they have high hopes for the new series. And all of these booksellers have something in common: they are located in cosmopolitan areas. This fact reinforces the publisher’s sales data; the series’ top markets are New York City, San Francisco, and Los Angeles, followed by Washington, D.C.; Boston; Seattle; and Chicago.

Weinstein is waiting until schools are back in session before sending Hunt out on tour. The author is scheduled to start a 10-city tour, primarily in the San Francisco Bay Area (where she now lives), in late September. The tour emphasizes school visits in partnership with local indie bookstores; currently, Hunt is slated to appear at 26 schools and nine stores. She is also set to appear at the Las Vegas Book Festival and at the Northern California Independent Booksellers Association annual trade show.

Caitlin Jordan, the children’s book buyer at Kepler’s Books in Menlo Park, Calif., praised the series for hitting an “untapped” market in beginning readers, ages seven to 10. She noted that she and her colleagues have observed a trend of parents buying multiple copies of the first two or three volumes to give away as birthday party favors. Jordan is planning to shelve the new series face-out, and, in October, the store is hosting an interactive author event with Hunt that will focus on cryptography.

Beth Thrall, children’s manager of Anderson’s Bookshop in Naperville, Ill. (another stop on Hunt’s tour), is also a big fan of Jack Stalwart. She said the series is one of the store’s “favorite go-to handsells for reluctant readers, especially boys in the second and third grades.”

Hunt has a number of events planned for Houston, where Blue Willow Bookshop will handle book sales for her three school visits. At Houston’s Murder by the Book, events coordinator John Kwiatkowski said the series has been a big hit since the staff discovered it and started stocking it in the store’s small children’s section. “The books just flew out of here,” he noted, explaining that there are very few mysteries for early readers. “It fills that gap. Parents who come to our store are looking for fun mysteries for their kids.”