The book starts out following the main protagonist, a young woman, as she leaves work, in California I think. Her car doesn't have air conditioning and I recall the the book referencing "4x65" or "65x4" AC, meaning driving 65 miles an hour with 5 windows open.
Eventually the woman gets transported to another world and is befriended by a wizard who is being chased and is unable to cast spells for fear of being tracked along ley lines. However, he is able to create spells using the woman's items, as this doesn't tap into the local energies and cannot be tracked. For example, he uses a battery to draw electricity and cast a lighting spell.
I remember the ending involved the antagonist wizard having created a pocket dimension and set up a computer inside of it, using the computer to "think" about the pocket world and keep it from collapsing when the antagonist wasn't there.