This concept of "discovering" new material always leaves a logic flaw for classes like the warlock or sorcerer where their abilities aren't "learned" but rather a natural part of their being. Warlocks don't have a "research" method of learning things. Sorcerers sort of do, but all it does is allow a method of acquiring spells - they never gain (shy of taking feats) additional spells above the number listed in the table.
If it was my game - I would have the player tell me what he wanted (from other books) and I would either approve it or not on a case basis. For the warlock the abilities gained probably would start to have some sort of physical effect (appearance wise) - especially if they were heritage feat base things.