You can listen online to a five part series on Radio 4 that examines four of Freud's works 100 years after they were first published. Each programme is only 15 minutes, but if you have to choose only two, then go for the first, 'Sexual Aberrations' and the last, 'Wit and its Relation to the Unconscious'.
The question of why we laugh is one of those enigmas that's always interesting to theorise on, but almost guaranteed never to provide a watertight answer. Still, Freud was one of the first to suggest that laughter was all about sharing our forbidden desires. We laugh because we're hearing the unsayable, the unpalatable. That accounts for 98% of my jokes anyway.
Radio 4: Freudian Slips