What kind of writing are you thinking of? There are books on every subject. Writer's Market lists markets and much info.

I have several books on writing for magazines and a few general publishing books. Most I picked up from the bookstore section on writing (with the dictionaries etc.). Some I found on the internet B&N/Amazon under 'writing' or 'non fiction writing'.
The magazine writing ones I have are by: Despain, Wrey and one by Writers Digest, with intro by Greene.
For general stuff though, Writers Market Companion covers most of the needs from getting projects organized thru publishing and marketing.

Are you looking for agricultural documentation that can be cited? Search the AgNIC portal.

Here is a link that might be useful: AgNIC Portal