I must have read his in the nineties. I think it was a series of books.

The first book starts off with a woman in a cabin in a modern-day American desert. I think a biker dude turns up, and then a wizard appears with a baby, and transports them to a parallel medieval world. The woman joins the city watch, protecting the baby, and the biker dude becomes an apprentice to the wizard.

I remember one scene where the biker dude casts a spell to change his appearance to a dung beetle, to sneak past some guards thinking it would be inconspicuous, but the locals had never seen a dung beetle before.

The book was written in English.