It bumps you to 105, but I was so rusty that I just started running around from the first book on. It's nice for doing book quests that need a fellowship since you get the skills of a 105 toon, some of the armor, potions, etc, so even if the area is scaled down, you still have some advantages. It's great for slayer deeding since I can one-shot everything in my path as I travel from place to place.
This guide is one of the best for explaining Aria of Valar and what you get and don't get, where to pick up additional class skill tomes for points, what order to do quests in and some little tricks I'd never know, etc. Note that this was written a while back, so things like stat caps are now outdated, but it still has some terrific tips:
http://dadislotroguides.com/aria-of-valar-trait-points-virtues-gear/
I highly recommend his youtube channel. His LOTRO life hack videos are amazingly helpful, especially for returning players. He's not playing LOTRO anymore (I do hope he comes back, however!), but the videos are fantastic.
I haven't even stepped into Mordor yet. I don't have the light/shadow thing figured out at all. I figured I'd level to 115/120 (or as high as I can get) in other areas first and then tackle that content.
Some handy links:
http://dadislotroguides.com/u21-mordor-lightshadow-system-radiance-2-0/
http://dadislotroguides.com/u21-mordor-gearing-for-success/