There are 2 books in the Gabi Mueller Series series. The newest book is Chasing Mona Lisa which came out in 2020.

1
The Swiss Courier

By Tricia Goyer , Mike Yorkey ,

Book cover of The Swiss Courier
It is August 1944 and the Gestapo is mercilessly rounding up suspected enemies of the Third Reich. When Joseph Engel, a German physicist working on the atomic bomb, finds that he is actually a Jew, adopted by Christian parents, he must flee for his life to neutral Switzerland. Gabi Mueller…
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2
Chasing Mona Lisa

By Mike Yorkey , Tricia Goyer ,

Book cover of Chasing Mona Lisa
It is August 1944 and Paris is on the cusp of liberation. As the soldiers of the Third Reich flee the Allied advance, they ravage the country, stealing countless pieces of art. Reichsmarschall Hermann Göring will stop at nothing to claim the most valuable one of all, the Mona Lisa,…