story identification - Need Help Identifying SF Book About Magic

I am trying to find a book I read many, many years ago.

The book starts with a princess, from a world with magic, coming through a rift into our world, without magic. She discovers, however, that one person in our world can "channel" magic to her, so that she can use magic in our world.

In the scene where this is discovered, the princess is attacked in the mundane-protagonists' house by assassins who also came through the rift. The mundane protagonist accidentally channels magic to the princess, resulting in an explosion that blows a hole in the floor.

That's all I remember... if you can help, I'd be very appreciative!

Edit: The mundane protagonist doesn't really know what she is doing, and has no fine control over the magic. I believe she started out, in the mundane world, performing general charms and cantrips and mystical-type stuff that doesn't actually work. (Similar to palmistry, etc.) However, the princess can sense that the mundane protagonist's spells actually DO have power. The mundane protagonist, however, has no idea what she is doing and no control - she cannot cast spells, and can't really "feel" any magic.

The princess's world is filled with magic that can be sensed all around, and used for spells. However, on Earth, no magic is available in the air and the princess cannot cast spells.

After discovering that the mundane protagonist has access to magic, even though she can't use it for anything, the princess asks her to "share" the magic, or feed it to her, so that the princess can use that magic to cast a spell. They try this, and the mundane protagonist pushes so much power to the princess that it supercharges the small spell that the princess cast, and blows a hole in the floor of the mundane protagonist's house.

Later in the book, the assassins arrive to try and kill the princess. The mundane protagonist and the princess are attacked in the front yard of the mundane protagonist's house. In desperation, they try the same channeling, and the princess is able to use the magic channeled by the mundane protagonist to defeat the assassins. Similar to the first try, this channeling results in a large explosion/damage to the front yard.

I hope that helps. Those scenes are really all I remember.