The Eternal Lover

A cliff-dwelling warrior of 100,000 years ago, Nu, is magically transported to the present, falls in love with Victoria Custer of Beatrice, Nebraska, the reincarnation of his lost lover Nat-ul, and the two are transported back to the Stone Age. The story is set in Africa, and the present-day sequences include Victoria's brother Barney Custer, protagonist of Burroughs's Ruritanian novel The Mad King, as well as Burroughs's iconic hero Tarzan from his Tarzan novels. (Wikipedia)