In case anyone is interested, here are a number of books/plays/poems (excluding Shakespeare) that various operas are based on. A fair few of them are rather different to the opera but they’re nonetheless incredibly interesting to read:
Kuzma Petrov-Vodkin - Sketches to the opera “Boris Godunov”, 1923.
The opera was written by Modest Mussorgsky in 1872 and first performed in 1874. It based on the historical tragedy by Alexander Pushkin in 1825. Boris Godunov himself was the first elected tsar of Russia who ruled after the Rurik dinasty distinction.
Boris Godunov is an opera by Modest Mussorgsky (1839–1881). The work was composed between 1868 and 1873 in Saint Petersburg, Russia. It is Mussorgsky’s only completed opera and is considered his masterpiece.Its subjects are the Russian ruler Boris Godunov, who reigned as Tsar (1598 to 1605) during the Time of Troubles, and his nemesis, the False Dmitriy (reigned 1605 to 1606). The Russian-language libretto was written by the composer, and is based on the drama Boris Godunov by Aleksandr Pushkin, and, in the Revised Version of 1872, on Nikolay Karamzin’s History of the Russian State.