Witcher games were never about completely explaining all things to the player... you could find a lot of info in the background, if you were careful..
look what they did with Alvin..you could find a letter from him, explaining some things from W1, yet that was completely random thing you could not even notice,if you were not careful...
At least they had a not so subtle hint.
But you're right, the books also never explained everything in detail. Although they only did not explain what had not to be explained, anyway. Yet they never left you wondering (At least not to the extent that one would assume, that the reason for it was just lazy writing.).