The Darwath Trilogy by Barbara Hambly
More specifically, the books are, The Time of the Dark, The Walls of Air, The Armies of Daylight.
The Time of the Dark I bought on my kindle for $2.99 or something like that and when I read it, I was hooked and had to get the rest.
Gil is a young woman who is entirely focused on getting her PhD in History. She teaches and tutors as part of the process. She begins to have dreams about some other place and time where the people run in terror from something unfathomable. These dreams have a very realistic and un-dreamlike quality except that she knows she is asleep and the people around her cannot see her. And then one night, they can. She appears and is questioned by an elderly man. When she answers that she was sleeping but now she's not, she falls back asleep and awakes in her own bed.
Gil and a biker artist named Rudy soon find themselves inadvertently pulled across the void into another world. A world where people run in terror from The Dark. Using their knowledge and experiences to help them cope and deal in a pre-industrial world not their own, Gil and Rudy must find a place in society and wait until it is safe for them to return to their own world.
Barbara Hambly draws from her own education in Medieval History to create a rich world for this series. Her characters are believable and full of life. The trilogy takes us through the battles with The Dark and delves into the long history of the world in which Gil and Rudy now reside.
There are two companion books that follow this series. One, The Mother of Winter, tells us of the world 5 years after the end of the trilogy. The other, Icefalcon's Quest, is the tale of a single character from the trilogy. I am currently ready Mother of Winter but I've had to stop a few times because it is creepy and I need a break at times.
I highly recommend this series for people who enjoy sci-fi fantasy fiction.