Responses to Kristi Stephens' query...
Dear Locus Online,
Maybe it was book by Daniel F. Galouye Simulacron-3 aka Counterfeit World on which 13'th Floor movie was loosely based. Details such as riding to the end of world are identical...
Best wishes,
Sergey Krasikov
http://www.svenlib.sandy.ru/pugovichki/
Dear Locus Online,
I'm pretty sure the novel Kristi Stephens is asking about is Daniel F. Galouye's Simulacron-3. As it happens, I just read that novel in the past couple of weeks. I liked it quite a bit.
Rich Horton
Dear Krissi Stephens,
Greetings from Reinbek, Germany.
The paperback your mother is looking for may be Counterfeit World (variant title Simulacron-3 [or Three]) by Daniel F. Galouye. It was published in 1964 and was filmed by Rainer Werner Fassbinder in 1973 - which may coincide with your mother's possession of a copy in the early 70's.
I'm not sure if the novel has been reprinted recently; perhaps the Locus datafiles are of use. However, a good second-hand SF bookstore should have a copy ready.
Kind regards and the best of luck,
Heiko Langhans
Dear Locus Online,
Regarding Kristi Stephens' query, the general theme she describes could be Simulacron - 3 by Daniel F. Galouye, which was filmed in the late 90s as The Thirteenth Floor. Having neither a copy of the book nor the film handy, I can't confirm how many of the details match. The novel was published in the 1960s, so it would have been available by the early 1970s time-frame she describes.
Richard Gilliam