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"Behind Enemy Lines," the sixth book in the Infinity Ring series, is by Utah author Jennifer A. Nielsen.

INFINITY RING, Book 6: Behind Enemy Lines,” by Jennifer A. Nielsen, Scholastic Press, $12.99, 186 pages (f) (ages 8-12)

Sera, Dak and Riq have time-warped with help from the Infinity Ring from Baghdad in "Cave of Wonders," the previous book in the series, to 1943 Scotland.

As Hystorians sent to change history, they realize they are to witness World War II as the means to rectify a break in the past. Before they can get their bearings, Nazi planes begin a blitz.

They are pushed into an air-raid shelter by a cadet who identifies himself as their contact. But he is killed trying to aid the people in the street. They are entirely on their own.

Deciding to spy on Nazi and Allied troops, they find themselves behind enemy lines; Dak in Germany seeking clues for their mission, Riq spying in British headquarters and Sera in Spain investigating a fake death planned to confuse the Nazi armies. Who would suspect three youngsters? They look and act completely innocent.

“Behind Enemy Lines” is the sixth book in the Infinity Ring series — each written by a different author — where three young sleuths use their skills to time-leap from era to era avoiding Time Wardens and SQ, whose plans are to take over the world. Above all they are trying to outwit Tilda, a dangerous SQ member who is “intent on clawing her way to the top.”

In the epilogue, Tilda’s threats are reiterated, assuring readers that Book 7 is not to be missed.

Utah author Jennifer A. Nielsen has woven a fast-paced thriller in “Behind Enemy Lines” with actual historical events such as locations and times of World War II bombings, rationing, Nazi war criminals and the Allies’ bogus plans to attack Greece to confuse the enemy. Tense moments combine into a series of “clinchers” that will assure fast page-turning.

Book 7, “The Iron Empire” — last in the series to be published in spring 2014 — promises to bring all the unsolved questions to the forefront such as the disappearance of Dak’s parents, Sera’s family’s loyalty and Dak’s eventual return to his own time period.

The books in The Infinity Ring series can be read independently, however reading them in sequence will add to the plot development. Each book has reading guides, maps and information about the website infinityring.scholastic.com that includes games and clues to the storyline.