There's a book called Servants in Husbandry in Early Modern England, by Ann Kussmaul.
Servants lived in the farm household and were hired by the year. Labourers lived in their own houses and were hired by the day. Servants tended to be hired in primarily pastoral areas, because sheep and cattle need care year-round, while labourers were used more in arable areas, since they had a few peak seasons in the year (planting and harvest) when they needed a lot of workers. Also, labourers were used more when population was growing (16th and 19th centuries) because then labour was readily available when needed. But in times of stagnant or falling population (late 17th to early 18th century) they tried to get more live-in servants in order to make sure they had a supply of labour when needed.