I would like to know if there has been any work relating legal code to complexity. In particular, suppose we have the decision problem "Given this law book and this particular set of circumstances, is the defendant guilty?" What complexity class does it belong to?

There are results that have proven that the card game Magic: the Gathering is both NP and Turing-complete so shouldn't similar results exist for legal code?

It's undecidable because a law book can include arbitrary logic. A silly example censorship law would be "it is illegal to publicize any computer program that does not halt". 

The reason results for MTG exist and are interesting is because it has a single fixed set of (mostly) unambiguous rules, unlike law which is ever changing, horribly localized and endlessly ambiguous. 
