The book was loosely based on the game, rather than the other way around.
The game is great (or at least, was) and ground breaking for its time. My favourite bit is you can actually leave Phlan and explore the North Moonsea coast; and cleanse the River which has an in game effect making the river clean. More modern RPGs should have instances where your actions actually change the game world permanently (for the better if you're successful of course!)>