Dead Cert by Dick Francis

Life is as normal when amateur jockey Alan York watches his friend Bill Davidson go over the jump in front of him to win the race on Admiral, but then the horse falls crushing his friend beneath him killing him and widowing his wife Scilla, leaving her with three children.  Alan goes back to the course and sees wire strung across the top of the fence and reporting it to the police engages with a dangerous underworld which revolves around love interest Kate Ellery-Penn, orphaned and raised by her upper class aunt and uncle and the said uncle's secret life.  Ends with a fantastic manhunt in the forests and woods around the racecourse with Alan on Admiral pitted against a gang of taxi drivers come tough guys with orders to kill.