Abe and Ann fall in love over a book and lose each other because of a letter, all between 1834 and 1835. Swoon.
Ann is the daughter of New Salem's cofounder. She's young, and she's beautiful. In other words, she's out of Abe's league. In this period, Abe is hunting vampires and has worked in two stores (that have both gone bankrupt) and has lost an election. So he really doesn't have anything going for him now, except for the fact that he loves books.
That's how Abe and Ann meet: they both borrow books from Mentor Graham (which should be a lesson to all of us: go to your library, meet your soul mate). As Graham remembers it, Abe "looked up from his book and was hit square in the heart by that ancient arrow. The two exchanged pleasantries, but I recall the conversation being one-sided, for Lincoln could hardly keep his wits about him—so struck was he by this lovely vision. So amazed was he by her love and knowledge of books" (6.67).
They fall in love in 1834, talking about Lord Byron, which makes a lot of sense. Byron is a romantic poet, and all the important women in Abe's life have encouraged him to read (by which we mean, his two moms).
But the problem is that Ann is already engaged to a guy named John MacNamar, who isn't really a gentleman. He's a scoundrel and a vampire. Uh oh.
But Abe doesn't know this, so he assumes that MacNamar is a stand-up guy, despite the fact that MacNamar already has shown himself to be a bit of a jerk by leaving Ann for three years and not dropping her a single line. So Abe writes a letter to MacNamar asking MacNamar to step aside so he could marry Ann and in response, MacNamar kills Ann (the same way Jack Barts kills Abe's mom—with a "fool's dose" of vampire blood).
Now, you might think that Abe would jump right back into vampire hunting after that, since he has both his mom and his Ann to avenge. But he actually takes a little vacay from the assassination business—albeit not a very fun one. After Ann's death, Abe decides that he's had too much death in his life: "I have given too much of myself already. Henceforth, I shall hunt only when it is convenient for me to do so, and only because it honors the memory of my angel mother... only because it honors Ann's memory" (7.10). And so Abe moves to Springfield and starts studying for a law degree.
But what if MacNamar hadn't killed Ann? (What if Abe had realized that MacNamar was a vampire and warned Ann in time?) Would Abe have married her and settled down? Would he have retired from killing vampires? It's very sad that Ann dies (not just for Abe, but for Ann, too), but her death seems to set Abe up for his next love and his future political career.