Lich King
Super Duper Whooper
Those are amazing books that I strongly recommend. The Last Guardian is a masterpiece, both written as such and has an enchanting developing story.
But to be honest, I do not find it as simple as you may do. Does the character has to be a visual encounter with first appearance in a game, or will it do for as long as the character is a matter of discussion in the game or his work is of significance to the game while that character himself made his first real appearance in a book.
Besides, the books originate from the game. The authors gives life to the lore Blizzard share with them. You are even able to find the content of the books in an official lore section at the official website of Warcraft, that brings up the characters and events that took place in the books (That are based on the games). Feels a little silly to be for example allowed to use Gul'Dan in a thread but not Ner'Zhul, his master.