What does a successful marriage, the sort where couples hold hands by the seaside at 60, need? Good sex, says author Shobhaa De, and great communication.

"All else - commitment, trust, faith - are givens if the first two work well," tells De in an interview.

De's latest book, her 14th, is "Spouse", all about the whys and wherefores of marriages.

From the time it hit the stands Tuesday, the book has already sold 10,000 copies making it an instant bestseller like all the others that De has penned. "I am proud of it," said De, married twice and mother of six.

India's best-known columnist, who is self-confessedly "obsessed with watching marriages", said even as the relationship changed over time, some things remained constant.

"The constants are - seeking love, seeking security, seeking companionship, seeking children," said De, adding that in all this there is also a changing pattern.

"The changing pattern intrigues me. Women are the drivers here. It is women who wish to rewrite the old rules of marriage. They are far more assertive in their articulation of key issues in modern marriage mainly - priorities, time management, financial control and space within the relationship."

But after decades of studying, analysing and contemplating about marriages, De admitted that it's impossible to know everything about a marriage.

"It is impossible to decode any marriage completely - even your own. Just as you believe you know all that there is to know about your partner, you discover areas that completely throw you!

"It is this unpredictability I find fascinating. Marriage in all its complexity remains one of life's great mysteries."

De has dabbled successfully in both fiction and non-fiction but contrary to popular belief, it is non-fiction that she said reflects her life.

"Non-fiction is more issues- and values-based. I put myself on the line and take a position when I write non-fiction, which reflects my beliefs.

"Marriage and motherhood remain non-negotiable in my book. Readers might be shocked and awestruck to discover a conservative wife and mother in real life. I love the surprise element of that," said De, who now wants to write a "definitive" biography.