word meaning "agriculture encyclopedia"

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

An example:

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.

I don't think there is a single word for an encyclopedic book on farming. Are you thinking about the Farmer's Almanac? Or perhaps a "Farmer's Handbook" is closer to what you are looking for? There are lots of Encyclopedias on farming as well.