Most of this book is centered on the Danish island on Bornholm in the Baltic Sea, which appears to have had a long history as a "sacred space" (over a thousand megalithic standing stones are spread around this relatively small island, and these are fairly rare in Scandinavia), and there are fifteen stone churches that date from around 1200ce, including four unusual "round" churches. Lincoln and his associates here spend a great deal of effort trying to chart out relationships between these various constructions, ending up with some extremely complicated geometry.
Now, I'm willing to admit that maybe I just "don't get" the obsession on these lines, angles, and repeated geometries, but it seems to me that the more convoluted the form gets, the more likely it is that somebody is trying to find some line that goes through something! This especially comes to mind when there are convenient omissions of buildings (at least two of the Bornholm churches don't make it into these graphs), and equally convenient inclusions of otherwise unremarkable locations, made special only in being at some point of intersecting lines, or being the center point for some form, or being the apex of an angle (heck, point C/111 here is several miles offshore with no solid object to anchor it!). Again, this may simply be a personal failing on my part (I have had many friends over the years who found this sort of mathematical game riveting), with my inability to work up enthusiasm for these "revelations" arising from my relatively "untutored" state for this sort of number play. Or not.
I will admit (ceding the author the benefit of the doubt on his numbers/measurements, here benefiting by a detailed charting of the island by the Danish government) that some of the "precision" involved is quite striking, assuming that one accepts the base premise that structure one is on this circle, and that structure two is on that line that intersects with something else and that structure three is on a facing of another shape within that circle, etc. Needless to say, I'd be MUCH more impressed if these structures were laid out even in an equilateral triangle, or square on the island, to the level of precision of distance and angle that's being proposed here ... but including one structure on each corner point. That would be something that I could look at and "get"!
One would think, then, that this book would be a profoundly disappointing read when ("spoiler" alert!) one finds that the "treasure" of Bornholm is this geometry/mathematics. Yep, the old "encoding knowledge for the ages in stone" trick. Lincoln argues reasonably convincingly that this is what the whole 10th-12th Century Bornholm thing was about. However, what saves the book (for me, at least) is the bits and pieces of background information regarding the Templars, and the tradition of "Swedish Freemasonry". I was unaware that the Baltic countries such as Estonia had remained strongly Pagan and were "inconvenient" to the Holy Roman Empire as they formed a choke point to trade ... the Templars were called in to aid in a long battle with these tribes. Also, there are threads here that the Templars had a "special relationship" with Swedish/Scandinavian secret groups which resulted in a surviving undercurrent beyond the 1312ce dissolution of the order by Rome. Obviously, the Templars (whose "round" church design is evident here) were very involved in the region, and it is interesting to contemplate how that influence came (as has been rumored in many other books) to shape the Masonic orders. There are also some additional interesting bits regarding the Templar Knights in Jerusalem, and some of the events in Southern France, with the Merovingians, etc., and recent research showing so-far-unexplored "subterranean chambers" existing under many of the discussed structures. Oh, one "geometric" thing of interest here ... Bornholm and Rennes-le-Chateau are "exactly" the same distance from Jerusalem, at an angle which suggests the point of a pentagram.
Anyway ... the version I have of The Templars' Secret Island is the Barnes & Nobel printing, which I got on clearance at the store (so I'm guessing it's out of print), but is still available on their web site. Deeply discounted copies can be found via Amazon's new/used vendors as well. Again, there is enough peripheral stuff in this book to make it well worth reading for somebody with as much of this material in one's library as I have, and I'm sure it would be fascinating to the "math game" inclined, and it's an interesting from a "Scandinavian history" level, but it just wasn't really "my cup of tea" (although I'm looking forward to further reading based on some of that "peripheral stuff"!) at this point ... however, as Dennis Miller would put it, "your mileage may vary"!