Comments and suggestions for additions or enhancements are greatly appreciated. Through the nineteenth century only a relatively small group of people made almost all of the important advances in mathematics, and collected works volumes exist for most of them. By the twentieth century more people were producing mathematics and the number of collected works proliferated. Collected works allow the researcher, historian or librarian to have much of the important literature available in a convenient and compact form. Sometimes collected works are the only way to find articles which were originally published in titles that are scarcely held even in the most comprehensive libraries.
Collected works are often very useful in deciphering obscure citations.
The best collected works may also include unpublished papers, correspondence, commentaries, translations, biographies, bibliographies, etc. The decision whether or not to include a particular author in this bibliography has sometimes been difficult. I have generally taken a broad view of mathematics, including statistics and applied mathematics. In some cases, titles that appeared by bibliographic description to be collected works upon examination turned out to be biographies, bibliographies, festschrifts, conference proceeding or some other sort of collection. In other cases the bibliographic description gave no clue, and only an in-hand examination revealed the title to be the authors collected works or correspondence.