If she's a lesbian, why is she reading a book about gay men (which I assumed from the Mars symbols).

Eriance said:
If she's a lesbian, why is she reading a book about gay men (which I assumed from the Mars symbols).

That's because she isn't a lesbian and has even had the desire to be Araragi's girlfriend if he wasn't already with Hitagi.

She's a self proclaimed lesbian fujoshi masochist, and either a compulsive liar, or someone who really enjoys messing with Araragi's head.
Point is, she's not that simple. Being a lesbian doesn't mean you can't enjoy yaoi. She might not even be reading it for the porn but rather the strong bonds between male protagonists.

It's Araragi, everyone is straight/gay for Araragi so it's totally natural to say your a lesbian but still want a piece of him!