The first part of the Monogatari Series.

There’s a girl at their school who is always ill. She routinely arrives late, leaves early, or doesn’t show up at all, and skips gym as a matter of course. She’s pretty, and the boys take to whispering that she’s a cloistered princess. As the self-described worst loser in her class soon finds out, they just don’t know what a monster she is. So begins a tale of mysterious maladies that are supernatural in origin yet deeply revealing of the human psyche, a set of case files as given to unexpected feeling as it is to irreverent humour.

(Source: Kodansha USA)

Note: The English release and the JP ebook edition have Bakemonogatari split across three volumes, instead of the original two volumes.