It's an old fashioned bodice ripper.
In it's time it was pioneering mainly due to the fact that the hero was vile. The heroine was duly raped, tortured and tormented for usually imagined wrong doings but called a romance due to the fact there was the HEA to look forward to. The hero was macho and took what he wanted.
Also the "love scenes" (or rather the violent love making or rape) was explicit which was new for romance therefore pioneering.
These books are not so much in vogue anymore but there are still some who like to see the heroine go through a lot of abuse before she gets her happy ever after so still a few fans about (quite a lot going by the forums and the amount that are available in digital format!)