If you like the music (which is undoubtedly since you came here, you probably had to see books that are called "Opera libretto". Illustrative of the content of many of the Russian, Soviet and foreign operas. Well, libretto by means content? No, it's inaccurate title. Actually libretto (Italian word libretto means book) - this is the full text of the musical stage works, that is, Opera, operetta.
Typically, compose the libretto specialized authors-librettists. Known in the history of musical theatre outstanding librettists, significantly influenced the development of Opera, such as P. Metastasio, P. Calzabigi, and later A. Boito in Italy, E. Scribe, A. Malak and L. Halevy in France. In Russia it was M. I. Tchaikovsky, who wrote the libretto for his brother P. I. Tchaikovsky, B. I. Belsky, who worked with N. A. Rimsky-Korsakov. Much of the libretto for Soviet composers was written by singer S. A. Caninum.
Often as the original libretto is a literary or dramatic work. Remember the most popular Opera: Eugene Onegin, the Queen of spades, "La Traviata", "Rigoletto", "Carmen", "the snow maiden", "Boris Godunov", "War and peace", "Katerina Ismailova". These are random Russian and foreign operas, based on famous works of Pushkin, Tolstoy, Leskov, Ostrovsky, mérimée, Hugo, Dumas son. However, these works have greatly changed, because the Opera genre has its own specifics. So, the text should be very concise: because the book word for much longer sounds than the spoken. In addition, the basis of the dramatic play is a dialogue. In the Opera should be arias, ensembles, choruses. All this also requires processing. Processing even dramatic plays.
If (elected by the story or novel, alterations even more: it reduces the number of characters allocated any one storyline and disappear entirely other. Compare, for example, "Eugene Onegin" by Pushkin and Opera Tchaikovsky, and you can easily make sure of that. Sometimes while changing the characters, even to some extent the idea works. Therefore, the composer sometimes criticised for distorting the intent of the writer. But such accusations of unjust: because the composer, together with librettist writes his own composition. As its essential component can be taken and any literary work, and historic event, and folk legend.
The libretto is an independent, not based on a literary work. The librettist or he composes his or creates, based on some documents, folklore sources, etc. So there was, for example, excellent, very original libretto N. Rimsky-Korsakov's "the legend of the invisible city of Kitezh and the maiden Fevroniya", written by C. Belsky.
Sometimes the author of the libretto is himself a composer. So Alexander porfiryevich Borodin on the basis of the great monument of ancient poetry "the Word about Igor's regiment" created the libretto of his Opera "Prince Igor". Himself wrote the libretto for "Boris Godunov" and of Khovanshchina modest Petrovich Mussorgsky, and in our time this tradition was continued by R. K. Shchedrin, who is not only the author of the music, but the author of the libretto of the Opera "Dead souls". Music history knows cases when the libretto by the composer chooses a complete dramatic work. That is, for example, "the Stone guest" Dargomyzhsky, written on the unmodified text of Pushkin's little tragedies.