My own favorites don't include anything about mountain biking.

My friend Andy Homan wrote an amazing tome called Life in the Slipstream; the Bobby Walthour Jr. Story. It's about Six-Day racing at the end of the 19th Century.

Andrew Ritchie wrote a very interesting book on the beginning of bicycle racing, from penny-farthings on the banked track to six-days in the early 20th Century. It's called Quest for Speed.