There is no one definite reliable source, which, to my mind, is one of the exciting things about magic history. People champion their own stories, things get hidden in the mists of time or in a dusty old book that no-one's read for years. For example, Erdnase's Two-Hand Transformation Number 1 has now been attributed to Houdini, and the one-hand weave shuffle has been demonstrated to have been invented by someone other than Howard de Courcey. Also, the extensive work of Marlo causes problems, as many of his published ideas were previously invented and published under different names.
I would suggest the best way to find the answer to any particular question would be to ask as many knowledgable magicians or magic historians as you can. Even if they don't know the answer, they may very well be able to point you in the right direction to find things out.
Subscribing to Genii is definitely a good idea though.