For me I think probably Don't Look Down by Jennifer Crusie and Bob Mayer. They wrote alternating chapters, with him writing the man and her writing the woman, which was interesting. It's an action/comedy/romance which is fun.
Oscar and Lucinda by Peter Carey is a wonderful book. Oscar is the son of a preacher, with hydrophobia and noisy knees, while Lucinda is a frizzy-haired heiress who buys a glass factory. Set in the early 19th century, Carey shares a witty, sharp story that deals with many issues of the time, and issues relevant to today - gambling being one of them. The mutual affection of the main characters shows that loneliness can bind people together. It's a delightfully awkward and whimsical book, I would have been happy if the story had never ended!