The Guardian runs Lloyd Shepherd's "top 10 weird histories," and it includes one of my favorite books, Neal Stephenson's Cryptonomicon.
Most of the books on the list are novels or graphic novels, but Shepherd also lists The Holy Blood and the Holy Grail by Baigent, Leigh and Lincoln, purportedly at least a work of nonfiction. Shepherd writes, "The core assertion - that Christ did not die on the cross, but sired a line of "children of God" who are protected to this day by a secret society - was taken by Brown as the engine of The Da Vinci Code, featuring a character called Leigh Teabing whose name, of course, is an amalgum of two of the authors of The Holy Bloody and the Holy Grail. Brown was sued by the authors for lifting whole parts of their book for his own; the presiding judge said this could not be plagiarism, as they had presented their book as a history, the facts of which should be available to other authors to use. The irony of this is so exquisite Christ himself would crack a smile."
Of course, Holy Blood also was an important source for Robert Anton Wilson's The Widow's Son -- one of Wilson's best books, but not commercially successful enough to inspire a lawsuit.