Almost anything by Steinbeck, with the possible exception of St. Katy The Virgin (A VERY funny and caustic comment on the Church - it is a short story from the collection, The Red Pony. HIGHLY recommended for a laugh-out-loud reading session). Also Tortilla Flats, and Cannary Row have a light-hearted and somewhat comedic feel to them, but even these latter two retain a subtle and overhanging pall of sadness.
“I can't read ten pages of Steinbeck without throwing up.”
- James Gould Cozzens, 1957
Also, as recently recommended by Fran, The Heart Is A Lonely Hunter can be viewed as a sad book in many ways but it also soars with brilliant writing and approaches the sublime as reading experience. I think I am safe to say, and I believe anyone who has read it would agree, that this book is a "must read". I have posted a short review in the Classic Books section of Reviews and Discussions about Specific Books and Series here on the forum for anyone who might be interested in reading it.
“I just got out of the hospital. I was in a speed reading accident. I hit a book mark and flew across the room.”
― Steven Wright