When Jilly Cooper, then a young ''Sunday Times'' journalist, was asked to write a book on marriage, she had been married to Leo Cooper for a mere seven years. Now they are celebrating their Golden Wedding, and although the institution of marriage has changed a great deal since this book was first written, much of Jilly's advice - frank, fearless, often hilarious, but always wise - still holds good. From the wedding and the honeymoon to life afterwards, including how to deal with the in-laws and how to tell if you spouse is having an affair, she dispenses anecdotes, jokes, common sense and endless optimism and fun.