By Masami Tsuda. Released in Japan by Hakusensha, serialized in the magazine LaLa. Released in North America by Tokyopop.
It's always rather refreshing to see these short, 1-2 volume series, as many of the frustrations you see in shoujo are not present simply as the series is so small. No agonizing about love confessions for volumes at a time, no 3rd party rivals, no hideous misunderstandings... just two people realizing that "Oh, that's what this feeling is, it's love".
Hazuki is the one to realize this, as he reads a famous poem that also describes the way he's currently feeling about Nanoha. Naturally, he starts blushing and stuttering, and basically becoming a wreck around Nanoha. But when called on it by her "monster", he pretty much comes right out and says he loves her. And from that point on, he ends up trying to be understanding and sweet, not pressing her on the issue and waiting for her to deal with her own feelings.
At first Nanoha is just as freaked out as he was, but when he says he doesn't mind keeping their relationship as it had been for now, she is fairly relieved. Sadly for him, this means she goes back to acting normally, which not only means not responding to his overture of love, but also that she is unthinkingly adorable and cute, sending him into a frenzy. He calls this her 'little devil', which makes an amusing counterpart to her own anthropomorphic 'monster', though it doesn't have an adorable SD-figure of it.
Speaking of the monster, things really begin to turn bad once Nanoha starts thinking about why Hazuki has fallen in love with her, and realizes that she's a plain, ordinary girl. (Being surrounded by her two pretty and poised friends hasn't helped much, though they both do their best to not have it be an issue.) After much hemming and hawing, she asks him what he ses in her, and he praises her forthright, honest nature, her ability to tell people exactly what she thinks of them... which HORRIFIES her, as she's always equated that part of herself with the 'monster', and now thinks that Hazuki doesn't love her, but it! (The shot of Nanoha realizing this is easily the funniest part of the book, by the way.)
After a talk with her friends, who reveal that they have their own 'monsters' inside them as well, Nanoha eventually works things out with Hazuki, and they end up as a couple. And then... well, then the story ends, as the main issue has been resolved, and this isn't an epic. This does make the final chapter a bit of an anticlimax, as it reads as just a chapter of the two of them being really happy together. Still, they're cute, and it does have a few funny bits where they note Nanoha is hitting a growth spurt.
Tsuda designed the series from the start to be 12 chapters long, one for each month of the year, and have it come out in 12 straight issues of LaLa magazine. As such, it's compact as I've noted, but that's also its strength as well. (Less of a strength is the 'month' thing... I never really got a sense that a year was passing, or each chapter was 1 month later. It felt more like it happened over a few weeks.) In the end, it's a cute, fun romance that does what Tsuda does best: highlight the thought processes of a boy and a girl, and how they may be very different things but end up working towards the same goal. And really, the couple is just so cute together.