Rita Lyttle wrote one pony book (under this name, at any rate.) She had a varied career; working first as a singer and songwriter on television shows. She loved horses herself, but only got one when she was 14. Her three daughters she was determined would get ponies earlier, as they were all pony mad. At the time she was writing Pony Madness, rather than working with horses, she was running a fashion business.
Pony Madness is a good, traditional pony story. A fatherless family move to a fairly derelict house in the country, and it conveniently turns out to have stables and fields. The girls eventually manage to persuade their cash strapped mother to help them loan a pony, and in the end they manage to buy the pony.
Finding the book
Fairly easy to find, and usually reasonably priced.
Sources and links
Dustjacket of Pony Madness