Sally Fielding wrote two books about Kate and her adventures on her grey half-Arab mare Shasta. The books are fast moving adventure stories: there is a Christian element to them, but it’s reasonably unobtrusive. The author does not bang you over the head with her faith, but does make it a realistic part of the story. There’s quite a difference between evangelistic books of this period and the 1960s: the use of opaque religious terms is dropped, and this use of everyday language makes the expressions of faith seem rather more normal.
Finding the books
Both titles are easy to find, and are very cheap.