Pa PITT is right the Stillwell ranch is amazing. That's amazing your mother knew her and that you have a signed book! I love that ranch because they let you drive anywhere; on road, off road, ditches, where ever. The only rule is not to go through a fence and to stay on the property. I go there to rock hound. You fill up your buckets full of fossils, agate, petrified wood, etc and pay by the pound.
On the trip we saw jackrabbits, javalina, deer, desert mice, road runners, who the hell knows how many birds, and a tarantula the size of my hand walking across the dirt road. The Milky Way was bright and the people were scarce. I prefer traveling with less people but it was a family trip. My parents have been taking my sister and I to Big Bend since we were kids and this was the first time my Dad got to go with his grandkids. For me it was a dream come true. Corny, I know but I've always wanted something other than a Jeep to go exploring these unimproved dirt roads...looking for new vistas to photograph then paint later. So little of the park is only seen from the main roads. You really need a high clearance 4x4 to see the majority of the park. It's undisturbed rough land.