Unlike the material trees that bear fruit only in season, this tree fulfills any desire instantly. Whatever a devotee wishes in the spirit of loving service to Krishna appears immediately — whether it be a seat, a meal, a flower, or a feeling of loving separation.
However, I cannot directly provide or link to a PDF file due to copyright restrictions. Many ISKCON publications, including stories about the desire tree (the kalpa-vriksha in the spiritual sky of Goloka Vrindavana), are copyrighted by the Bhaktivedanta Book Trust (BBT).
The roots of this tree are made of devotion. Its trunk is spiritual ecstasy. Its branches are pure love of God. Its leaves and flowers are all the virtues of the soul. And the fruits are the direct reciprocation of love between Krishna and His devotees.