I read about 15 years ago a book about teleportation devices and alternate histories, and I have the greatest difficulties to recall its name or its author.
The story goes more or less like this: government-funded research yielded functional teleportation devices (booths), and as a first human try, they send an operative with documents from one facility to another. When the guy arrives on the other side, his personality seems to have changed: he has full-blown paranoia and wants confirmation of the identity of the people he is talking to, and finally commits suicide rather than giving away the documents.
It is later understood that the devices are not for teleportation, but rather doors to alternate earths, with alternate histories. You could even put nothing in it, and get something in return.