The Golem Mastery book can be found in the library in The Ruins of Varsoon where the Tome of Life spawns. You can get to this book with invis, or sneak – unless the named is up.
The Orc Mastery book can be found behind a shield in the tower at the crossroads in the Commonlands.
The Siren Mastery book can be located in the Enchanted Lands, at the house in the docks – check the cupboard on the desk!
The Bixie Mastery book can be found in an instance inside Rivervale called The Drafling tower – check the bookcases!
The Centaur Mastery book can be found in the Thundermist Village in Thundering Steppes, examine the bunkbeds!
The Elemental Mastery book has been found in 2 locations – in the Academy of Arcane Science in North Freeport, and the Mage Tower in South Qeynos.
The Evil Eye Mastery book can be found near the forge in Runnyeye.
The Ghost Mastery book can be found on the first floor of Nektropos castle, on a bookshelf in a room of manservants.
The Zombie Mastery book can be found in an urn in Fallen Gate, in the area called Market 2 – check near the tents!