This book is a fantasy book about a man who is pulled into another world where he has magic powers. He's pretty much a 90-pound weakling in jogging pants and sweats, since he was jogging in California when he gets pulled in. He's brought by this other man, also from Earth, as his replacement. This man has been there longer and gained more power. He doesn't really believe in magic, but starts believing when washing his face in a river cleans his contacts better than contact cleaner fluid. He sets off to visit this other wizard, who lives a long way away on/in a mountain. There's a funny scene where he takes out his contact lens and the members of his party scream, convinced he's plucked out his eye.
Wizard! You have plucked out your eye!
There's a bit of romance between he and a woman, where he learns that women have a spell cast on them to prevent disease and pregnancy and at a wedding, the priest removes this spell. Since they're not married, this means he can have worry-free sex with her. There may be some demons sent by an evil wizard.
Eventually they get where they are going and something happens. The other wizard is killed by this evil wizard, I think, before he can send him back. At the end, he learns that there are other ways back, made both easy and hard because his power means that magic and magical artefacts are attracted to him. He sets off for this lake, supposedly caused when some demonic rubies fell out of the sky. The gems are still supposedly there.
May have been the start of a series.
The start had a fairly detailed map.