I'm actually a bit lost on this one – my usual thing is literary fiction or big fantasy series, and I've never properly tackled the Mills & Boon catalogue. I recognize they're the heart of the romance genre, especially with all those Harlequin imprints, but I couldn't name a single definitive title. The whole 'must-read' idea for them seems tricky. Aren't they more about the experience and the trope you're in the mood for than a specific classic list? Like, if you want a sheikh story or a medical drama or a Regency, you'd search for that modifier rather than a 'best of' list. I feel like their strength is in consistency and delivering on a very specific promise. Maybe that's why fans gobble them up; you know exactly what you're getting. For a true must-read, I'd probably look at authors who started there and moved to bigger single-title romances, like maybe Nora Roberts? But that feels like cheating the question.
I did try one once, plucked from a charity shop because the cover was so gloriously dramatic. It was fine, a perfectly pleasant way to spend an afternoon, but it didn't leave a mark the way a romance novel with more page count and deeper character work might. So my uneducated take is that the 'must-read' is less about the book and more about understanding the formula itself – how they build tension, resolve conflict, and satisfy that craving for a guaranteed happy ending in under two hundred pages. It’s a craft study, in a way.