Like much in the publishing industry, Mills and Boon novels are around for a very long time. Long before the raunchier bondage of Fifty Shades, Gerard Mills and Charles Boon founded the company in 1908, initially as a mainstream publisher. It wasnt until the 1930's that they began to concentrate solely on romance, mostly in hardback, ( with brown covers that became their trademark long before the more familiar pinks and pastels of later generations). 

However, it was the paperback boom of the post war era that saw Mills and Boon explode sales-wise. As far back as the 1940's and 1950's a number of (still prominent) themes were already well established - the doctor/medical love affair being one, the Regency Romance being another . 

Colour coding came into play at this time, as well as the rose symbol that ( still ) is associated with the brand.

http://thebookshop.ie/mills-boon-the-burning-sands...

Later additions in the 1980's saw a ( slightly) more risque profile , with greater emphasis on sexuality , at least in some of the monthly series. Australian and Western sub genres were created. 

http://thebookshop.ie/mills-boon-romance-hunters-m...

In response to the Sexy Vampires popularity brought on by Stephanie Meyer and the Twilight series, Mills and Boon created the Nocturne sub genre , with a darker, almost YA feel . 

http://thebookshop.ie/mills-boon-nocturne-lone-wol...

Other spin- offs flourished also, some briefly, as the genre is nothing if not diverse. In the USA, the Harlequin Romance imprint is a huge publishing concern, and by the 1980's and 1990's,  a huge range of Mills and Boon novels were produced, with as many as 20-30 new titles per month, as well as specials, triple deckers, ( 3 in 1 novels) and reprints. Despite this, many authors, especially from the 1980's can be hard to find , one of the main reasons being that when books were returned from bookshops each month, the covers would be ripped off the books and returned for credit, and the books themselves pulped or binned.

Colour coding still applies, the newer books are more contemporary in feel, and have less obvious traditional Mills and Boon branding. 

We get a lot of Mills and Boon books, and they remain great sellers. They can sometimes attract a certain amount of snobbishness ( even among booksellers who should know better) - as being viewed as something less than Literature with a capital L. Well, we say feck that. If something has worked and thrived sales-wise for over 100 years, its doing something right. 

To see our large range of mills and Boon, from a wide range of eras, just search under Mills and Boon - they're all together in a section - at 

http://thebookshop.ie/fiction/mills-boon/