Brendan,
The highest of the "Catskill Mountains" (technically they are not mountains in the geological sense) are in Ulster Co., but parts are also in Sullivan (the old "Borscht Belt"), Delaware, Greene and Schoharie Cos.
An interesting historical subject is the New York City Reservoir System. To form the Catskill and Delaware Systems, massive blocks of land were seized by eminent domain and flooded.
Another is the "Hardenburgh [or "bergh"] Patent". In 1707, a Mr. Hardenburgh acquired 2 million acres of land, encompassing the Catskills, from an Indian tribe for 60 pounds. Probably a crooked deal.