Fruit and Vegetables for Scotland: What to Grow and How to Grow It beat a long list of 24 other titles to win the category, including tomes from television gardeners Alan Titchmarsh, Monty Don and James Wong.
The judges called it "a practical book with personality … meticulously researched and impressively informative. It fills a long-empty gap for Scottish gardeners coping with weather conditions that are vastly different to the rest of the UK."
Cox said: "I was pretty shocked when our names were read out, having seen the list of famous names we were competing against. It is very satisfying that the judges appreciated why Scotland’s conditions are so different from those further south, and we are very proud to be honoured."
Cox is one of the world’s experts on rhododendrons and an author of 11 books on gardening and horticulture. He is managing director of his family firm at Glendoick, near Perth. His previous Birlinn title won the guild's Garden Reference Book of the Year in 2009.
Caroline Beaton works part-time in a therapeutic garden in Perth.