The El Dorado Adventure is the second of four other adventure books in the series. These include The Illyrian Adventure, The Drackenberg Adventure, The Jedera Adventure, and The Philadelphia Adventure.

The Illyrian Adventure presents Vesper and Brinnie's trip to Illyria during an economic and political situation similar to that between Greece and Turkey at the end of the Ottoman Empire. After she finds an archaeological treasure, Vesper makes peace between the two opposing cultures but gains the lasting enmity of Dr. Helvitius, whose schemes of economic exploitation are spoiled.

In The Drackenberg Adventure, Vesper, Brinnie, and Brinnie's wife Mary visit the Grand Duchy of Drackenberg, a quasi-German province in Europe. During the diamond jubilee celebration of Drackenberg's ruler, Vesper discovers that Dr. Helvitius is plotting against the grand duchess with her nephew and heir. Vesper solves Drackenberg's problems with help from a roving gypsy band and...