Ultimate Guaiguaipi
That book is no less powerful than his God tier passive skill. He can literally create balance breaking skills and equipment. Now him not getting more skills of other classes cause of not choosing a class doesn't matter. He can create all top tier passive skills of other classes and then have them enhanced by God tier passive skill. Now that's just another level of broken bug character.
Arthas is not clever. Unless it was left out of the translation, the terms were vague. He never actually said he would give him the book. He said "a reward" and brought out the book. The reward could have been opportunities to have Arthas use Law on Mike's behalf. And Mike not giving any kind of an out or implicit compromise... Arthas is clearly unhappy, and Mike knows this game isn't going to stay just a game. I'm not saying drop it; a bet was made. But unlike the dumb same/similar level player wagering what he can't afford, it's perfectly reasonable of an NPC at dozens of times his level to assume he isn't actually a threat.