A South African educational book published on October 1, 1991 states that Nelson Mandela died on July 23, 1991.
The book snippet that was found on Google Books says:
Yet, words like reconciliation and the new South Africa simply were not long enough to bridge the vast differences between blacks and whites. The chaos that erupted in the ranks of the ANC when Nelson Mandela died on the 23rd of July 1991 brought the January 29th 1991 Inkatha-ANC peace accord to nothing. It lacked one thing. It lacked the last realisation.
Book Title: English Alive, 1990: writings from High Schools in Southern Africa
Authors: Kathleen Heugh, Anita Kennet
Date of Publication: 1 Oct 1991
Page No.: 54
Publisher: Claremont: Western Cape Branch of the South African Council for English Education, 1991
ISBN: 0620151862 9780620151863
OCLC Number: 123781891
Credits: Timeline Shift.
The book can be found on Google Books & WorldCat.