On a trip to her boyfriend's family ranch in Montana, Caitlin finds her relationship with Jed Michaels threatened by his ruthless former girlfriend and by the prospects of the two of them being separated by college
Francine Paula Pascal was an American author best known for her Sweet Valley series of young adult novels. Sweet Valley High, the backbone of the collection, was made into a television series, which led to several spin-offs, including The Unicorn Club and Sweet Valley University. Although most of these books were published in the 1980s and 1990s, they remained so popular that several titles were re-released decades later.
After Sweet Valley High came the Caitlin trilogy, followed a few more books; I think I bought most of them in the States, and read this sometime in 1987 to 89, I think. The Caitlin series were a bit more daring the SVH books, just a bit. Think PG-13.