The Secret of Secrets by Dan Brown

Accompanying celebrated academic, Katherine Solomon, to a lecture she's been invited to give in Prague, Robert Langdon's world spirals out of control when she disappears without trace from their hotel room. Far from home and well out of his comfort zone, Langdon must pit his wits against forces unknown to recover the woman he loves. But Prague is an old and dangerous city, steeped in folklore and mystery. For over two thousand years, the tides of history have washed back and forth over it, leaving behind echoes of everything that has gone before. Little can Langdon know that he is being stalked by a spectre from that dark past. He must use all of his arcane knowledge to decipher the world around him before he too is consumed by the rings of treachery and deception that have swallowed Katherine.