They're DM materials, so they should go wherever such things are found.
So, assuming the standard PHB/DMG/MM model, they should go in the DMG. If there's a single Core Rulebook, they should go in the DM's section of that book.
One thing I would be interested in would be an "Encyclopedia Version" of the game, though, where the whole game is presented by topics. In which case, the magic items would have their own book(s). But that's obviously quite a departure from what has gone before. (Not to mention that the internet has pretty much killed encyclopedia and taken their stuff!)