I haven't read Cycler, the first book of this duology, as I have not come across it in person yet. I'm pretty sure this book review will spoil the end of the previous book, though, so most of it's going to go below the spoiler cut. I'll just set up the premise here first:
Upon hitting puberty, Jill McTeague developed a freaky problem. Some time before, during, or after her period*, for about 4 days, Jill grows a penis and turns into a guy. I should be more specific about that: Jill does not stay herself, with her own personality that just happens to have a penis now. Oh no. She has an entirely different male personality inhabiting her body, Jack. During their teen years, Jill had to be officially out of school four days a month, and Jack was kept locked up by their parents in a room with nothing to do but read books and watch porn. Jack has not grown up with much socialization, and he's pretty much a spy to boot. He remembers everything Jill does and has fallen in love with her best friend, Ramie. It's mutual. Meanwhile, Jill's interested in a bisexual boy, Tommy, who Jack can't stand. I'm not sure why Jack can't stand him, but it seems implied the bisexuality wigs him out under the circumstances.
And now I'll go below the spoiler cut.
* Not having a copy of this book myself to check, I browsed the Internet trying to get this clarified. Alas, I read different answers to this question all over the place. My best guess is that this takes place before, as some kind of super weird PMS symptom.