Complex sentence: Many country-specific stamp catalogs include postal stationery in their listings and there are many books devoted to the postal stationery of individual countries, however, the principal encyclopedic work is the nineteen volume Higgins & Gage World Postal Stationery Catalog.
Split into simpler sentences: Many country-specific stamp catalogs include postal stationery in their listings and there are many books devoted to the postal stationery of individual countries. The current principal encyclopedic work is the nineteen volume Higgins & Gage World Postal Stationery Catalog.