I'll just say that I don't think this is really a spoiler since you see Cailan as king from the get-go.  
 
So does anyone know how Maric died?  If you haven't read the books and don't want a minor spoiler for that... 

As far as I know, his death is never addressed in the game.  I haven't really looked at the codex stuff, so it may be in there.
 
I didn't know that there was a 2nd book, thanks for the head's up.

2nd book? I just have to first one. What is the second one called?

The Calling, it is basically what Grey Wardens call it when their time has come and they venture into the Deep Roads if I remember correctly.

The second book involves dungeon crawling of Grey Wardens and Maric into the Deep Roads.

Also if you really want to get to know the character of Duncan, I highly recommend reading The Calling.

It doesn't seem like Maric was a big player when the game was originally written, but when Gaider decided to go back and write the books he needed a subject/character an chose Maric. Now we have this really wierd situation because the books build him up as a main character only to have that completely reversed to the point where his name is mentioned like 5 times in the game. I thought we would at least see his ghost in the fade or something.