Source book, statistics of income.
Sole proprietorship returns

APA Citation

United States. Internal Revenue Service. (1987). Source book, statistics of income. [Washington, D.C.?]: Dept. of the Treasury, Internal Revenue Service.

MLA Citation

United States. Internal Revenue Service. Source Book, Statistics of Income. [Washington, D.C.?]: Dept. of the Treasury, Internal Revenue Service, 1987.

Warning: These citations may not always be complete (especially for serials).