I learned about The Templars’ Secret Island (New York: Barnes & Noble, 2002) from watching “Holy Grail in America.” In this book Erling Haagensen and Henry Lincoln try to tie together medieval churches on the Danish island Bornholm, the Knights Templar, the Holy Sepulchre, the Temple of Solomon, and the Ark of the Covenant.
They hypothesize that “Bornholm was laid out [by the Knights Templar] with absolute precision as a teaching aid. . . . Here would the student come to learn, and to learn not merely the mathematics and the gemoetry. Here also could be taught the underlying symbolism” (132). In other words, the student would learn the secret knowledge the Knights Templar uncovered and retrieved from Jerusalem.
While their hypothesis is intriguing, I think their extrapolation of geometric figures from the positions of the churches on Bornholm is based on too few points and requires more evidence.
They do list some interesting books in their “Bibliography”: