From the Foreward by the Editors ...

When Professor Otto Dunkel died in 1951, it was found that he left a bequest to the Mathematical Association of America. After deliberation, the Board of Governors of the Association voted that the income from Professor Dunkel's bequest be spent in publishing an Otto Dunkel Memorial Problem Book, based upon the better problems that have appeared over the years in the American Mathematical Monthly. This was a very fitting decision, for Professor Dunkel had served for twenty-eight years as an editor of the Monthly Problem Department and is the one chiefly responsible for the Department's growth and high standards.

...

In this connection, the listing of the 251 still unsolved Monthly problems for the given period may be of some interest and may stimulate some graduate and undergraduate research.