Books related to popular anime?

I've started actually reading books more often lately, and I was wondering if you guys had any suggestions for books that have similar storylines to popular anime. I'm looking for something that has a similar plot to things like Code Geass, Gundam Seed, Legend of Legendary Heroes, etc. Basically something where it starts out with two people who were close at a young age, and then due to whatever circumstances were separated and joined opposing factions and were later forced to confront one another on the battlefield. Multiple suggestions would be very much welcome and appreciated.


EDIT: Movie/series suggestions would be appreciated as well!

You should read Mary Stewart's retelling of the legend of King Arthur and Merlin. The first book is called The Crystal Cave. Kind of fits your requirement but it's not based on anime, which to me is a silly basis since Japanese story-telling is somewhat rigid and limited in imagination.

True it doesn't really matter to me whether it is based on anime; it's more about the plot of how two people were close at a young age were separated and now had to confront each other (or something along those lines). The reason I mentioned those three anime is because that is where I got the idea from and they are what sparked my interest in such storylines (Lelouch vs. Suzaku, Kira vs. Athren, Ryner(roland) vs. Kiefer(Gastark).)

I wouldn't say a young age, but the Night Watch series by Sergei Lukyanenko has a similar element to that. The lead protagonist, Anton, is a member of the Night Watch while a young friend and neighbor of his, Kostya, is a vampire and therefore destined for the Day Watch. Kostya is not usually a focus in the majority of the novels, but it might be what you're looking for. He is mentioned frequently enough, and the growing idealogical conflict between the two is clear.

(I'm trying to avoid spoilers here, so I'm being intentionally vague and ambiguous.)

@Ryllharu

Thank you for the avoidance of spoilers, it will make the read more interesting. I will add that to my list with Animeniax's suggestion. Please keep the suggestions coming!

I'm not too sure about the stories itself but maybe the Mechwarrior franchise has some stories like that

you might want to look into game franchise in general since there are plenty of stories like that

too bad that there is no book about Metal Fatigue (game).. at least I haven't heard about that, but that would fit perfectly

3 brothers, each one joining a different corp.