Christine Feldman provides a good introduction to the role of joy in meditation and spiritual practice. Much of the article is about effort and Feldman defines skillful effort as effort that brings a sense of joy, so it’s not grim effort. You can find her article here: Making a Joyful Effort
Jack Kornfield has a pair of articles on his website on natural joy that are taken from his book, The Wise Heart. He speaks of the grim effort he undertook during the first several years he practiced in Asia. However, he eventually realized that he had been ignoring the joy reflected in the Buddhist cultures he was living in, and the joy and laughter of his favorite teachers. Seeing this, he began to emphasize the cultivation of joy in his practice. You can find the two articles here: Natural Joy and Natural Joy Part Two