Fifty Shades of Grey, Fifty Shades Darker, and Fifty Shades Freed, written by British author E L James, have topped bestseller lists internationally.
They have spent 16 weeks atop the New York Times' list and the first book in the series has spent 10 weeks at No 1 in New Zealand.
Wellington City Libraries collection team leader John Stears said while the books were slow to take off, they currently had 71 reserves for the three copies of the first in the series. Libraries were buying more copies to accommodate demand.
The books feature explicit scenes of kinky sex using bondage and discipline, or BDSM, as it describes the relationship between a young virginal college graduate and a business magnate with a penchant for whips and collars.
Kiwi sex toy retailer D.Vice director Wendy Lee said interest in sex toys had dramatically increased due to the series. Their website now featured a "Fifty Shades of Grey" section, where the "light bondage" items used in the book, are listed together, including whips, collars and handcuffs.
"It's kind of like, although in quite a different way, what Sex and the City was for vibrators. In it the characters were talking about vibrators and using them. It changed how people, particularly women, viewed vibrators - it made them OK."
Romance and erotica has always been popular, but the degree of sexually explicit material is growing.
"If you go back 30 years, commercially published romances were generally much more mild," Hachette New Zealand managing director Kevin Chapman said.
In 2008, Walus wrote A Slave of My Own Desire, an erotic fantasy novel with BDSM scenes.
She sent it to Red Rose Publishers in the United States who "ummed and ahhed over it, even though the BDSM angle is very light".
Her next books were more "sweet romance" than kinky fetish sex and sold much better, she said.
Much has changed since then, and Walus believed it was the timing of the Fifty Shades series, which led to its blockbuster success.
"It ties in with the fact that violence is a lot more acceptable now than five or 10 years ago.
"If you look at the types of things people watch on TV, there's a lot of crime, real crime and criminal procedures, so we've become desensitised about violence.
"BDSM is intrinsically not violent. It's different to rape or assault but people tend to see it in the same light. If you find real violence acceptable, violence in private lives is acceptable too."
She predicts a growth in the number of increasingly risque books coming out in the next few months - and the growth in the number of authors who want to write them.
Next month, the Romance Writers of New Zealand are holding their annual conference and have invited Walus to speak about writing erotic sex.
"Up until quite recently [erotic fiction] was frowned upon [by romance writers], it was an uncomfortable topic. [Now] writers are opening up to that level of kinkiness and maybe there's an increased market following Fifty Shades."
More at the Sunday Star Times