Do what you feel. But if you do give it to her, fade back a little bit so she doesn't feel obligated or crowded. And don't make a big deal about it. But, the next time you get her something I'd make sure it wasn't as valuable monetarily. That way she understands that while you're not cheap, you're not going to be spending money on her willy nilly style. If she's hot like you say she is, guys are always trying impress her with spending money and trying get her every little thing her heart desires. Don't be one of them. She'll respect you more if you're not.
Personally, I'd get her something cheaper but equally as thoughtful this B-day. Then hold on to the book and surprise her with it when things are a little more serious between you two. She'll aprecite you and it more.