Final answer:
The Mental Measurements Yearbook, edited by O.K. Buros in 1939, is a significant reference for evaluating psychological and educational tests and is key for professionals in human assessment.
Step-by-step explanation:
The best reference book on tests which was originally edited by O.K. Buros in 1939 is widely recognized as the Mental Measurements Yearbook. This comprehensive guide has been instrumental in the field of psychology and education, providing critical evaluations of tests, including those for educational levels, intelligence, aptitude, and personality. The Yearbook continues to be updated and remains a valuable resource for professionals involved in the assessment of human abilities.