The publisher is making all titles available as e-books every month, with the exception of its special releases, centenary products and summer sale books.
They can be read on PDAs, such as the Blackberry, computers and special e-readers, which are designed to replicate the experience of reading a book.
Mills & Boon admits that one of the benefits is that fans can read the books in "total anonymity", allowing them to avoid the embarrassment factor. It also says that it will mean fans won't have to wait for the latest titles to be delivered, as they can be downloaded instantly.
Waterstone's is the first UK book retailer to make a major push in the e-book market by encouraging publishers to make digital versions of some of their books available for sale.
Sony's e-reader is already available and being sold in Waterstone's. Amazon has its own version, the Kindle, which is heavily promoted on its US website, but not yet available in the UK.
Sony's version, which retails for £199, has had a largely positive reaction from users, although Waterstone's retailing of e-books has come in for criticism because of the pricing and the search engine.
Many of the books cost more in digital than the paper edition, with publishers cautious of eating in to their bread-and-butter revenue stream by making cheap digital versions available.
Waterstone's is retailing the electronic version of the new James Bond book, 'Devil May Care', at £15.19, compared with the publisher's hardback list price of £18.99. Waterstone's is selling the book online for £11.39 and Amazon.co.uk for £7.59.
Mills & Boon's e-books start at £2.99, which is the same price as the paperback versions sold through its website.