Eff is a thirteenth child. And a twin -- her brother Lan is that fortunate thing, a seventh son of a seventh son, unlike the misfortune of a thirteenth child. Her aunts and uncles and cousins are so disapproving and mean-spirited about it that her parents take off so her father can teach magic at a land-grant college near the Great Barrier.
Because she lives in an alternate world. Columbia -- not America -- is filled to the brim with all sorts of monsters. Some are (merely!) Ice Age creatures like the saber-toothed tiger and the woolly rhinoceros, but others, like the steam dragon, are magical. It's after the Secession War, a hundred years after independence, and people are just inching past the Mammoth River into the Great Plains. (Query: how did the Civil War happen on schedule without California's Gold Rush? Ah, well, Eff does not go in depth on history and its causes.) Lewis and Clark vanished without a trace, as has every other expedition trying to go west.
But they go west and live at the college, and Eff goes to school and learns different types of magic and gets sick and there are the Rationalists, who don't use magic, and her mother going back to help when the first grandchildren are born, and William, the only child of the other professor, and a steamdragon. . . lots of things. 'cause this is not an epic fantasy. It's a Bildungsroman about Eff's growing up. She gets involved in studies about magical things, leading to her working on something, but it's not the point of the story.