So after ripping off Baigent and Leigh's Holy Blood, Holy Grail to write The Da Vinci Code, Dan Brown is back, ripping off Baigent and Leigh's The Temple and the Lodge to write The Lost Symbol.
Seriously, though, Baigent and Leigh published their books as nonfiction, therefore any fiction based on it is fair game, just as I can write a science fiction based on science research or fantasy based on archaelogical or historical research.
However, if Baigent and Leigh had admitted their work was fictional in nature rather than scholarly, who could honestly say that Dan Brown didn't steal all their best ideas? And he's still doing it.
Dan, thy name is R.F.Chutzpah.