Located in Abysmal Sea at 49, 104, 116
Located in Crescent Reach at -1417, -1487, 33 :: 2nd floor south side by tailoring merchants
Located in Plane of Knowledge at -146, 710, 4 :: Cubbyhole south of entrance to library building
They are in the Find Window (Ctrl+F) as Tradeskill Books category for their respective zones.
Requires you to have 300 skill in tailoring in order to see the items on the merchant.