Early on, Graham Hancock used to add a paragraph at the end of each chapter of his books wherein he stated clearly what was verified fact and what was speculation. I gather that this has faded away for quite a long time now. Of course I have not read his books for a very long time.
I read "Magicians of the Gods" recently - a few months ago maybe. His books can be interesting, but need to be read as speculative fiction - much the same as "Holy Blood, Holy Grail" or the Illuminati books. T.Lobsang Rampa's Third Eye books and Carlos Castenda's books are in the same kind of category.
They are not great literature and not at all scientific, but the point of them is to kick the imagination to question things from a different place. Sometimes this is helpful. Some of Hancock's speculation/interpretation on Gobekli Tepe has been supported by professional archaeologists in a study - specifically that they include a record of a comet strike that had a major impact on humans:
https://phys.org/news/2017-04-ancient-stone-pillars-clues-comet.html Also his underwater explorations are in sync with understanding of post ice-age sea level rise, and archaeologists have been finding quite a lot underwater, over many years now.
Science does not take a straight unerring path. In the 70's and '80s there was a lot of argument about multi-regional human evolution as opposed to out of Africa. This was, to all extents and purposes, won by the out of Africa side, but is now swinging back to the multi-regional or, at least, a combination.
Jean M Auel's "Clan of the Cave Bear" series was fiction, and contains many laughably impossible descriptions. But, some of her imaginings are now mainstream, including Neanderthal/modern human interbreeding.
Unfortunately Dan Brown is not in the same league as any of these. It is fiction and he knows his readership and how to produce books with enough big scenes, characters and plot twists to keep them entertained. That he gets his ideas out of the aether (he himself is not proposing anything profound or interesting) and has a terrible writing style does not matter. In fact, I would say he is writing for the screen, to get hit movies, not for serious readers.