Singed!

The epic tale of how Twitch rinses and becomes the most feared hero of all the land, hardly anyone can face him. Then through nerf monsters cast a spell on him to a walking site ward.

But seriously Jax. Jax is referenced in DotA in an item. So I think he has the background to be the one they follow.

A book would hopefully have multiple protagonists and tell the story from many perspectives (preferably from the angle of each faction; Bandle City, Ionia, Bilgewater, Noxus, Demacia, etc.). Garen, Kat, Ashe, Trynd, and Trist would be good options for the main character of their respective factions, as they fit the role, but the protagonist role is interchangable. The champs affiliated with each faction would probably be supporting characters (Cass and Swain for Noxus, Lux and Xin for Demacia, Teemo and perhaps Veigar for Bandle City, etc.). The Void story would probably be covered as well, so Cho, Kog, Malz, and Kass would be the center of attention. Hopefully many of the independent champs (Fiddle, Morde, Vlad, etc.) would get their story covered as well, and many mysterious champs like Jax would have their secrets revealed.

Very cool thread - I think that most likely answer is (as some people have said) that they would write from multiple perspectives, in a third-person omniscient perspective, or try to tell a complete story through separate short stories that could be told from a variety of perspectives.

Even more important, I think, would be the conflict around which the book would revolve. Would they use a previously developed source of conflict, such as a historical tale of the Rune Wars and the formation of the League, or use a current source of conflict such as the ever-encroaching Void? Would they instead invent a new source of conflict, contrived specifically for the book?

The "main character(s)" would be largely be chosen by the conflict of the story arc. Personally, would love to see something from the perspective of the Adjudicator, the person/people that judge the champions as they join the league. Man, my mind is racing now, I'm so inspired! Might go out and write this myself...

tldr; too bad.

Another interesting suggestion is that a new champion would be conceptualized specifically for story purposes. This said champion though would not appear in gameplay because his journey has been told, his goals have been achieved, he has no need to enter/stay in the League.

I think it's quite obvious that the shopkeeper is the main character. Going off on an adventure when the shopkeeping life becomes to much of a bore, he is THE mover and shaker of Runterra.

Garen because he has the very heroic stereotype in him with Ezreal or Lux as the secondary protagonist.
Villain probably Veigar or Karthus.

Villain-wise, Malzahar would be the closest the game's world has to an overarching supervillain.