Where have all the divas gone?; The Last Prima Donnas, by Lanfranco Rasponi. New York: Alfred A. Knopf. 635 pp.

''The Last Prima Donnas'' is a spellbinding document of a bygone era and one of the most important opera books to be published in many years. Lanfranco Rasponi, in his long career as a writer and public relations man, has accumulated interviews with 55 of opera's great ladies and gathered those pieces under one thick cover. Though the title sounds cataclysmic, it is, tragically, on the mark.

The book gives the reader a vivid taste of a bygone era and serves simultaneously as a textbook study in why the world of opera is in crisis today. In these 600 or so pages of text, Mr. Rasponi offers firsthand accounts of these singers' careers.

This is not the place to undertake a discussion of why the world of opera no longer produces great voices or great personalities. It need simply be stated, as it is in the author's introduction, that when Leonie Rysanek, Leontyne Price, Joan Sutherland, and Montserrat Caballe cease to grace our stages, there will be virtually no one left of that special breed.

A common theme in all the interviews is the singers' holy consecration to their task. They talk of the tremendous personal sacrifice a public career necessitated, of the responsibility of the singer to protect and sustain her instrument despite the pressures from conductors, composers, and the public.

Mr. Rasponi, in a recent conversation, observed sadly, ''The great sense of responsibility is gone. How are you going to go on with no major singers - there are practically none, and those that are left are in deep trouble.''

The names in the book are a who's who of the opera world from the turn of the century through the mid-1960s. Such well-known divas as Kirsten Flagstad, Lotte Lehmann, Dorothy Kirsten, Grace Moore, Amelita Galli-Curci, Renata Tebaldi, and Lily Pons are included. Numerous others such as Maria Carbone, Germana di Giulio , Giulia Tess, Ester Mazzoleni are hardly household names, but had seminal influence on opera in Europe. All lament the lack of serious purpose in the young singers they watch or teach today.

I asked Mr. Rasponi why he set out to interview all these subjects: ''I felt it was important. . . . Nothing else has been written about them, and when they are gone, that knowledge will be gone, too.''

''The Last Prima Donnas'' is not only remarkable reading; it is the sort of book any opera lover will want to annotate and keep on the reference shelf.