Why doesn’t the woman buy the book?

M: What’s that book you just picked up?
W: The sociology text professor Smith uses in his course.
M: You had better read it if you want to pass the course.
W: But it is too expensive. I simply can’t afford it.
M: How much does it cost?
W: It costs 40 dollars.
M: Did you check the used book section here? Maybe they have one.
W: No, they don’t. I have asked.
M: Why don’t you get it from the library?
W: Are you joking? I’ve been trying for months and the book is always out. There are more than 45 students in the course and every single one wants the book.
M: Listen, you know my roommate, Henry, don’t you? He took the same course last year and I believe he owns the book. I’ll ask him if he’ll lend it to you.
W: Oh, Tom, that would settle everything. That’s very kind of you.
M: My pleasure.