There are 3 books in the The One Dollar Horse series. The newest book is Fire Storm which came out in 2014.

1
The One Dollar Horse

Book cover of The One Dollar Horse
A thriller set in the equestrian world about making the impossible possible, about reaching the top on a one dollar horse. Fifteen year old Casey Blue lives in East London's grimmest tower block and volunteers at a local riding school, but her dream is to win the world's greatest Three…
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2
Race the Wind

Book cover of Race the Wind
The second romantic thriller in the gripping One Dollar Horse equestrian thriller series in which Casey and her horse Storm face the challenge of the Kentucky Three Day Event. From the prize-winning author of the BLUE PETER BOOK OF THE YEAR AWARD, the second romantic thriller in the gripping One…

3
Fire Storm

Book cover of Fire Storm
What if victory cost you your life? The final novel in THE ONE DOLLAR HORSE series by prize-winning Lauren St John, in which Casey and Storm compete at the Burghley Horse Trials. Teenage eventing star Casey Blue has it all - fame, her champion horse Storm and a boyfriend who…