Books would be amazing, but I think comics would be the best option.
I'm not sure if they're still being put out, but I remember a few years ago World of Warcraft had a few mini-series that gave a bit of backstory to the world. I read two of them, one about the founding of the Argent Crusade and one about the origin of the Gilnean Worgen curse. They had good art, decent writing, and told a lot about the behind-the-scenes of the world. To those that read them, they were a great source of information and entertainment, and they also did a lot of world-building. I don't play WoW anymore, but I'm pretty sure I still have those comics.
If Riot were to make comics -or even write books- about Valoran and the champions in it, I'm positive there would be a big market for them. Comics are really popular right now, and seeing League of Legends comics might actually pull more people into the game. Example: A person goes to their favourite comic shop to pick up the new Thor or Ultimate Avengers or Saga comic, and they see a comic for League of Legends on the 'new arrivals' shelf. The person has never played League, but has heard of it. The comic's art looks cool so they pick it up and start reading it. They like the story and even though they decide not to buy the comic right away, when they go home they'll download the game and check it out. They decide that if they like it, they'll go back and buy the comic.
But Riot, if you do decide to do any print-format stories for League of Legends, I have but one small request... Please have at least one arc/story about Viktor and the Glorious Evolution. Thank you. ^_^
EDIT: Oooohh... Accidentally made this into a wall of Text. Tl;dr: Comics = good. League comics = good + more $$ for Riot.