Temple Trouble by H. Beam Piper

I'll bet you did not know that our little earth is not limited to the single time line on which we happen to exist. That's right; There are actually thousands, no many millions of parallel times, each existing alongside all the rest, using the same real estate, but following their own path. Some extremely primitive, some very advanced, but all blissfully unaware of the others. Of course this does not affect us at all, unless ... unless one of these time lines discovers the existence of the others and then a way to move easily back and forth from one to the other. Then a few problems might happen. Well, it has happened and the ParaPolice are kept busy sorting out things like what happened here on an earth time line where Yat-Zur, the 6 armed god, battles Muz-Azin, the Crocodile god ...