An official canon source would be like a liscensed Star Wars novel, guidebook, movie reference, recent George Lucas quote, etc. Not another game, not some wiki page (none of which are official), not the databank.
A lot of the game stuff becomes murky because of the game mechanics that are there by nature. Comic books exaggerate things (as does the Clone Wars series). So it's not that such things can't contain canon, just that it's far easier to see it elsewhere. It's much easier to read things into games when we take them out of the game context (like one person interpreted Dark Forces as showing that Kyle Katarn always had high force ability because he used the Force to carry all those heavy weapons in the first game!).
Of course when you're talking about game stuff you also have to check it for accuracy. For example I think the NEG to Characters said that Kyle Katarn spared the life of Sariss. However in the game and the novelle he clearly kills her (the author probably got her confused with Tavion, who is dead anyway later on after the book was written).