Quick history...
The asterix is usually used as a wildcard in programming. You put a * there to mean any value you can think of.
A while ago, someone realized that you could call the Fighters handbook, the Wizard's handbook, etc (fighters/wizards/etc)books, or *books in computer parlance. (Well, not actually. It wasn't with the old D&D books. I think it started being used with White Wolf Clan/Tribe/Tradition/etc books, but I might be mistaken. Same general idea, though.)
And since it's hard to say "asterix" in regular speech, computer nerds referring to the asterix in verbal communication called it a "splat", similar to how they called the exclamation point "bang". So the *book, or splatbook name managed to stick from there.