The Land of Painted Caves

While creating a toy for a crippled child, Ayla inadvertantly invents a crude combustion engine, saving the tribe from total anhilation at the hands of nature. She is congratulated by her best friend the lion and conveys her satisfaction to her horse, whose language she speaks. She celebrates the tribe's deliverance from the peril that only just began to occur shortly before her arrival by having perfect sex with her sensitive cave-man lover.

It's the sixth book in the series, thirty years after the first book was printed and ten since the last. Jean M. Auel clearly needs a new jetski.