Part of my research that I find very important and enjoyable is studying maps of the areas I set my books. I have spent hours pouring over the smallest details on printed maps that I've managed to find drawn from the eras I write. By having maps of those eras at hand, I am able to send my characters down the correct roads, across the right rivers, and climb the named moors and mountains of the area. To my characters, who have lived in that area they need to know the places, roads and rivers as well as if they actually lived there, as do I.
A good map will always be of valuable use to a writer, and in turn, that information will be of great benefit to the story and hopefully make it more enjoyable and real for the reader.
For example, in my book, To Gain What's Lost, which is set near Leeds, West Yorkshire, I have found maps of 1870 to help me get a feel of the area my characters would travel.