I read the books but there are no rules for this: What about if two (or more) ships are the same grid in space battle? With which fire arc could fire each other?

All of them, or just from the incoming direction... or?

If both the ships are in the same VU, well that's rather close. I'd call it as whatever arc they would be in were they in adjacent VUs. In this case you have to consider where ship came from and where it's pointed to figure it out. Most of the time when they end up in the same VU they're probably going to ram though.