Legal Question in Civil Litigation in California

It is not really surprising that you don't find any authority. If it isn't expressly authorized or prohibited by statute, it is highly unlikely that anyone would go to the trouble and expense of making a reportable appellate case out of it. There is no question as to the court's authority to issue subpoenas to the third party, and they don't otherwise "have a dog in the fight," meaning no standing to object and appeal. No judge of sound mind would do it if it materially prejudiced one of the parties. So it is extremely unlikely, bordering on impossible, that an appellate court would even issue the peremptory writ, much less rule against the trial judge. In short, taking that order up on a writ would be utterly quixotic and a waste of money. AND even if such a case was taken up and heard, it would likely be limited to its facts and not suitable for a published opinion.

You can be ordered, pursuant to subpoena to produce documents. I would start researching based on subpoenas.