I found this shopping at Half Price Books, bought it, then confirmed my suspicion:
Barrett, Neal. Through Darkest America. Worldwide, 1988. First paperback edition, a Fine- copy with tiny bump to bottom front corner and slight age-darkening to paper, otherwise apparently unread. Inscribed by Barrett, Jr. to fellow Texas science fiction writer Lewis Shiner: “11/29/88//Unca Lew—/Having you here for/a while has made my/day, as ever—/(Signature)”.
Lew confirmed that it was indeed inscribed to him, noting he must have sold it because he was moving again and already had the hardback…