The book is part of the Fifty Shades trilogy by E.L. James and delves into the world of erotica, with explicit descriptions of sex and erotic love.
This week a bedroom survey of over-50s revealed naughty sex and a morning tumble were favourites among older lovers.
About 700 Kiwis nearing their golden years participated in the survey, performed by online social club and lifestyle magazine GrownUps.
Many are indulging in the pleasures of the flesh on a daily basis, with others choosing to save it for "special occasions".
Sex toys are also popular.
More than 30 per cent of those surveyed nominated the morning as the preferred time for getting intimate and about half said sex toys featured in their bedroom antics.
Dr Christine Roke, national medical adviser at Family Planning, said people of all ages enjoyed getting busy in the bedroom.
"Even in rest homes and retirement villages, people are sexual beings and they need to have privacy. Sexual feelings are there for life."
But findings that suggested nearly one-in-five older lovers had never used a condom had to be taken with a grain of salt, she said.
"This is a survey that has been done by a fun social club, and people who took part tend to be quite social."
It was important to remember that the older generation grew up without condoms or today's heightened awareness around safe sex.
There had been a slight rise in the number of sexually transmitted infections in people aged over 50, which could be related to New Zealand's high divorce rate and more casual relationships, Dr Roke said.
Last year, 8551 divorces were granted, according to Statistics NZ. This translated to a divorce rate of 9.8 for every 1000 Kiwi marriages.
One-in-three couples who married in 1986 also divorced in 2011 before reaching their silver wedding anniversary.
Meanwhile, survey results reveal a lucky 4 per cent of 50-plus Kiwis are having sex at least once a day. But that's a distant dream for 11 per cent of those surveyed, who admitted they never had sex. A further 14 per cent had sex only on special occasions.
GrownUps website founder Richard Poole said the results showed how important it was to practise safe sex.
"It's quite obvious that the over-50s are having a lot of sex, but it's important to make sure you know your partner has had a 'sexual warrant of fitness'."
Getting lucky over 50
4 per cent - at least once a day
11.5 per cent - never
14.3 per cent - on "special occasions"
51.1 per cent - use sex toys
38 per cent - don't need sexual performance enhancers such as Viagra
NZ Divorce rate
2011: 9.8*
2010: 10.2*
2009: 10.2*
*(per 1000 marriages)
