The godless man : a mystery of Alexander the Great / Paul Doherty.

Enhanced descriptions from Syndetics:

It is now 334 B.C., and Alexander has smashed the Persian armies at the Battle of the Granicus and is roaming the western Persian Empire like a hungry predator, living up to his nickname of "the Wolf of Macedon." Arriving in one of his prizes, the great city of Ephesus, the success of his campaign is threatened by a series of violent murders brought about by a high-ranking Persian spy known only as "the Centaur." Worse, one of Alexander's old tutors, Leonidas, is found floating face down in a stagnant pond at the House of Medusa--and this doesn't look like an accident, since the House of Medusa is linked with a guild of assassins who formerly flourished in Ephesus. So once again Alexander's friend and physician, Telamon, must set about unraveling this swirling mass of blood-strewn mysteries, this time working hand in hand with the king's eerie Master of Secrets, Aristander. As always, one of the biggest obstacles is the volatile and unpredictable nature of Alexander himself, a consummate actor whose lust for power and glory matches the carnage and intrigue that dog his footsteps like the Furies themselves.