The English National Opera/Royal Opera House opera guides have generally made excellent slim volume introductions to operas of the standard repertoire, and also more offbeat work. This volume on the first 4 operas of Michael Tippett, which all concerned at the time thought would be the only operas that he would compose, fits well with that tradition. Complete libretti are included, as well as insightful introductions and footnotes, as well as a discography up to the time, 1985. Some might find the print a bit small, especially in the lengthy stage directions, but the volumes are compact, after all.
On one small point, I will admit that I rather strongly disagree with Paul Driver's assertion at the start of his essay on "The Midsummer Marriage" that he would recommend that opera to "someone innocent of the genre or sceptical about it", because of the obscurities and general oddness of the libretto, which impression Driver is eager to sweep aside and dismiss. I wouldn't want to introduce someone to opera with a work as texturally arcane as "The Midsummer Marriage", even though the music is splendid. However, this is a difference of opinion merely, and not a comment on the quality of Driver's essay. Since this volume is essentially in honor of a composer who was still alive at the time, the harsher criticisms that have been leveled at Tippett's libretti, such as what consider a cringing or embarassing use of then-contemporary slang (e.g. "Oh boy", "Play it cool", "Ain't that so?", "Burn, baby, burn") over the years have been generally omitted.
The "unintentionally complete" part of the title here is because Tippett completed one more opera, "New Year", in the late 1980's. At the time of the publication of this volume, Tippett had indicated that he would not compose any further operas after his 1977 work "The Ice Break", but obviously things turned out otherwise.