English National Opera Guides are ideal companions to the opera. They provide stimulating introductory articles together with the complete text of each opera in English and the original where appropriate. Although it is impossible to trace any one particular theme running through the operas of Michael Tippett, the libretti of his four operas are fascinating to compare. The dense allusions of "The Midsummer Marriage" (1955), here annotated, gave way to the classical formality of "King Priam" (1962); the psycho-analytical preoccupations of "The Knot Garden" (1970) hardly foreshadow the contemporary political commentary of "The Ice Break" (1977). Each work breaks new ground and provokes unexpected responses. The libretti offer unique introductions to the music. They incidentally throw a searching light on the direction of British theatre since 1945.