Most of Stalin's major essays, articles and speeches from the 1920s and 1930s appeared in two anthologies: On the Opposition and Problems of Leninism. (The links are to the table of contents for each collection.) The first of these contains 26 documents dating from 1921 to 1927, while the second consists of 28 texts from 1924 to 1939. (Two pieces, The October Revolution and the Tactics of the Russian Communists (1924) and Concerning Questions of Leninism (1926) appear in both collections.)
Although all but 5 of these 52 documents (those dating from 1935-39) can be found in Stalin's Works (which cover the years 1901-1934), I have nevertheless prepared the on-line version of the texts contained in these two collections based on the 1974 Chinese edition of On the Opposition and the 1976 Chinese edition of Problems of Leninism. The Chinese editions are themselves based on the English translations of the various texts as they appear in Stalin's Works (1953-55), "with some technical changes" or "minor changes . . . in the translation", and they include the endnotes contained in the Works. The precise nature of the "technical changes" is not clear, but in comparing some of the texts, one occasionaly finds that where elipses appear in the extracts from Lenin's pieces in the texts from Stalin's Works, the Chinese editions frequently include a few more of Lenin's words where the elipses are placed.
I have chosen to follow the Chinese editions for only one reason: their editorial work provides an immence service to the reader. The texts as they appear in Stalin's Works have one serious drawback: in citing the material that Stalin quotes from Lenin's writings, the editors of Stalin's Works provide the reader with only a volume and page number, and very rarely the title of the piece by Lenin from which the material was taken. The problem, for the contempoary reader, is this: the references are to the 4th Russian edition of Lenin's Works . The service the Chinese editors have provided is to identify, in footnotes, the specific title of Lenin's text in all such cases. The on-line version of each document from the two anthologies includes, as a reminder to the reader, the "Publisher's Note" detailing their preparation of the texts.