The 'prevailing' mood is somber. The mood changes throughout the book, though. At times it is lighthearted and positive reflecting Morrie's personality. The prevailing mood is somber or solemn because we know from the very first page Morrie is going to die. Which this is not positive-but Morrie makes light of his foretold death, and the memories that Morrie and Mitch Albom shared contribute to a more positive mood.