Book about government-funded research yielding functional teleportation that causes personality changes

Question

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.

Answer 1

This is almost certainly The Infinitive of Go, by John Brunner. From the plot summary:

For the first significant test using a live person, a diplomatic agent is Posted to a foreign embassy from the USA. The test is an abject failure: the agent, George Gunther, is unexpectedly armed and also demands a countersign upon arriving at his destination. Since nobody was instructed to provide him such a countersign, Gunther immediately assumes he's been intercepted by the enemy and shoots himself, triggering a destructive failsafe in the package of documents he was carrying. It is assumed that the Posting affected the agent's sanity. Faced with termination of the project, Dr. Justin Williams, the inventor of the technology, arranges to have himself Posted from the same embassy back to his research laboratory.

The alternate universe understanding does come later:

The problem is this: the Poster links two congruent spaces, but it searches many universes to find the best match. When all factors are accounted for, including a person's state of mind and the state of the machine itself, the best match is more likely to be found in a machine from another universe. This is how a Dr. Landini arrived that is a man descended from baboons but speaking English and coming from a world with almost exactly the same history as the one on which he arrived. Although evolution took a different track on his world, the outcome was almost exactly the same as on the world where he arrived; had it been different, he would have gone to a different universe. The person who first comes to understand this is not a scientist, but a philosopher.