Wanted to say that I've finished reading your book – read it intensively over a two-day period. I found it both easy to read and thoroughly enjoyable. Because it reads much like a detective novel, it carries the reader along with it... I found your writing style to be incredibly lucid and clear: perhaps this is due to you being free from the usual over-poetic digress of those wanting either to obscure the reader or seem overly professional. You also engage the reader on your personal quest with passages of your own travels, thoughts, and vistas, which was enjoyable and helped to place the reader inside of your story.
You have indeed done quiet some detective work! The analysis of the winter solstice and its attendant angles was impressive: the work on the picture reminded me of a book by Elizabeth von Buren, who also spends a great deal of time on the painting clues, as well as other paintings, noticeably by da Vinci who she brings into the picture. Saying this, you do stick very firmly 'on the ground', which is a credit to your angle as it shows you have no interest in the occult aspect – in fact, the book shows that your interest lies in deciphering the clues rather than worrying about what the treasure itself may be... as your book says, perhaps a combination of both physical treasure and hidden knowledge.
Your intro to the religious history of the area was also clear and gave a good background.