Complex sentence: The original edition was engraved in aquatint and hand-colored by Robert Havell Sr. and son, Robert Havell Jr.; known as the "Double Elephant folio", it is often regarded as the greatest picture book ever produced.
Split into simpler sentences: The original edition was engraved in aquatint and hand-colored by the two and produced what is known as the "Double Elephant folio". It is often regarded as the greatest picture book ever produced.