I am not sure quite how to describe what I mean by "formula" here. I have read hundreds of romance novels over the years, and some of them, like Jane Eyre, are clearly in romance genre but do not come across as formula romance -- even though they contain many of the tropes that formula romance relies on. Austen is generally more romantic comedy than romance, when you get down to it. This is not a matter of modern vs classic: Shades of Milk and Honey was a romance novel, but I wouldn't call it formula romance. Slightly Dangerous is formula romance. It has the familiar elements: the tall handsome athletic male protagonist obsessively attracted to beautiful-in-nonstandard-way female protagonist, who is equally attracted to him. Sex. Many pages devoted to exploring the protagonists' inner lives and feelings, especially their feelings for one another. Male and female protagonist who are clearly destined for each other from the start and get married by the end.
I am not saying this as a slam against formula romance. I like formula romance, although it can get repetitive.
Formula romance novelists these days do something I didn't notice when I was reading the genre a lot as a teen: the common-setting series. Formula romance doesn't lend itself to series, because the formula is "one couple per book, who will live Happily Ever After at the end". To get around this, the common-setting series uses one of the supporting characters from the previous books as half of the couple for the next book, while protagonists from previous books appear as supporting characters in the next. Slightly Dangerous turned out to be the last in a series of six books about six siblings. In the common-setting series, you can always recognize which are matches from previous books because they're the likable, happily-married characters who are surprisingly young and attractive, and at least one half of every couple is rich. Often they have very young children. Two-thirds of the way through Slightly Dangerous, there is a dizzying cast reunion of characters I don't know and have a hard time telling apart plotting to unite the book's current set of characters. It's actually pretty cute. I strongly suspect that people who read the series from the start would enjoy the book more than I did, in part because the cold, brooding male protagonist of Slightly Dangerous must've made a heck of a supporting character.
One of my difficulties with romance as a genre is that a lot of common tropes that other readers think are romantic, I dislike. Jealous or possessive lovers. Love triangles. One of my least favorite is the basis for Slightly Dangerous: "Hate at First Sight". For most of the book, the protagonists' primary feelings towards each other are lust, dislike, irritation, annoyance, and contempt. Not necessarily in that order. The novel borrows key plot points, certain character traits, and some dialogue bits direct from Pride and Prejudice, with everything turned up to 11 and/or adjusted for modern tastes. The male protagonist is extra broody with a troubled childhood. The female protagonist -- who is charming, cheerful, good-humored and lovable when seen from the PoV of others -- is also broody when it comes to her inner life: she's often described as laughing at herself and appearing unphased when she's feeling mortified and miserable. This is ... rather a shame: I really like her when viewed from the outside, but viewed from the inside she sometimes comes across as fake. Also, she has a Dark Secret. Did I mention he's a duke and she's a penniless widow and a schoolmaster's daughter? One of the funny things about regency romances by modern authors is that they're often stuffed with upper nobility, while authors like Austen and Bronte rarely have characters with a title at all. (I can't think of a single Austen protagonist who had a title, in fact, although some of them are the children of baronets or knights, which are the bottom ranks of the aristocracy). I suspect some of their less-classic contemporaries may've been freer with titled characters, though.
I have some more nitpicks and would not recommend this particular book unless you actually like "Hate at First Sight"* as a trope. Nonetheless, it's a pretty smooth, fun read. The protagonists are entertaining despite their trope-induced-flaws. There are a number of silly and amusing escapades. Also, having most of the duke's family plotting to get him together with the penniless widow is a charming reversal of trope. I'll give another of Balogh's books a try, although I'll scrutinize the description better in the hopes of not getting another that relies on tropes I don't care for. On the scale of "how much I liked it", though, I'm going to have to give it a 5 of 10.
... in other news, the library has three more books for me. THE HYDRA IS GROWING EVEN FASTER. I tried to pick them up from the library this afternoon, but forgot that it closes at 5PM Friday & Saturday. Not that I don't have more books at home to read anyway.
* Yes, Pride and Prejudice also uses the "Hate at First Sight" trope, and I love P&P anyway. Even tropes I dislike can be done so well that they're fun: Austen is the exception that proves the rule. It helps a lot that (a) Elizabeth has good reasons for disliking Darcy from the start, and good reasons for getting over her dislike and (b) Darcy doesn't so much dislike Elizabeth at first sight as consider her beneath his notice, and changes his opinion quickly once he learns more about her.