A county plat book is a public record of land ownership within a given county. It comes in the form of maps that delineate the borders of sections of land in that county.
A plat book contains a list of owners of every piece of land in the county that is 5 acres or larger; land that is smaller than 5 acres is considered to be too small to record. The maps in a plat book show where the townships begin and end. Plat books tend to be updated about every 3 to 5 years and are useful because they give township information regardless of who owns the land at a given time.