ICAO produces a lot of information about international operations. Start with ICAO Annex 2 and Annex 6 Part Two.

ICAO member states issue Aeronautical Information Publications (AIPs) to inform foreign operators how to fly in their territories legally. When researching a new country to fly into, the sections of high priority are…

Crossing the Atlantic along the Blue Spruce Routes is best accomplished between FL200 and FL280. Below FL200, you start to lose VHF communication and radar coverage. At FL290 and above, additional navigation, communication, and surveillance equipment requirements and authorizations (LOAs) are needed to operate legally. 

A guide to operational procedures across the North Atlantic Airspace.