Hi
An Ag Lab was an Agricultural Labourer - i.e. any labouring work to do with a farm - usually a married man, employed on either a regular or a casual basis, but who lived elswhere (i.e not in the farm house).
It was next to the lowest occupation in social classification, and as such went unrecorded apart from the type of work they did - i.e labouring. But this country was built on that type of person.
There are lots of books that touch on Ag Labs and Agricultural History, but no specialist book on Ag Labs themselves; but you could try to find a copy of these:
Farmworkers - A Social and Economic History 1770 - 1980 by Alan Armstong (1988)
Annals of the Labouring Poor; Social Change and Agrarian England 1660 - 1900 by KDM Snell, (1985)
Hope that helps
Chris in 1066Land