The last word of the Dragon Aspect Shout is located in Apocrypha.
Do the main campaign, and you will get a Black Book called "Waking Dreams". Read it to teleport to a realm where you will have to find four more books and place them on the pedestals. These books have to be placed in the correct order - match pictures with names of books.
The tower in the center will then glow green. Go there and read Chapter VI and it will take you to the area where the last word is. There are some enemies there to defeat, but otherwise just go to the wall with the inscribed symbols to learn the shout:
WORD OF POWER LEARNED - WYRM, DRAGON ASPECT