Shadowman by Della Galton
Karen and Rob run a stable yard. They're in financial difficulties, and then someone starts sending anonymous letters accusing Rob of having affairs. Karen needs to find out whether they're true and who's sending them, as well as saving her marriage and their business.
This is a wonderful little book - the kind you can read in a couple of hours on a rainy Sunday, and it'll keep you totally engrossed. Lots of twists and turns and dead ends, as you're led first one way and then another wondering who dunnit. Highly recommended.