I am looking for a single word that means "agriculture encyclopedia"; a book about farming. Does such a word exist?
Not a single word, but along the lines suggested by @DianeEngelstad, how about agronomy handbook? From M-W:
agronomy: a branch of agriculture dealing with field-crop production and soil management
The Agronomy Handbook is a comprehensive resource that discusses soil properties, plant essential nutrients, soil test results, and recommendations. Agriculture-related analyses are indispensable in supplying accurate, current information for making management decisions regarding soil fertility and plant nutrients.