about ME3 : the new ending show some images depending on what you did and who you helped, not that much exciting stuff.
about the book, Diablo is indeed a good example :
you can play the game and fully understand what the story is about without reading the book but the book gives you more informations about some points of Diablo's universe and some small event in the game refer to those books but this is not a big deal if you have not read its. and the trilogy is awesome but so is Cain's book and I am really looking forward Tyrael's book and the new novel.
Books about Rivellon should indeed be a good idea, but they really need a good author. Basic example, I do not remember who wrote the book about Arthas but it sucked sooooo much...
