G., can I give you a bit of advice from someone who has been there/done that? (My kid was the one complaining about other kids, btw...)
Please keep in mind that while some good books might be nice, this may be a long journey that kids are going to need to learn on their own. What is 'friendly' behavior and what isn't; what makes a good 'friend' and how to be a good friend, how to stand up for oneself, etc.
I agree with some roleplaying, but one very strong caution I want to offer-- and I am not saying you are doing this, by the way-- is to not make more of this moment of snotty behavior than it warrants. It's easy for us as parents to want to 'fix' situations of relational aggression, or to ask "how" so and so treated our kid, etc. But we also want to help them --even more-- develop a sense of resilience.
Which means that we temper our own responses and make sure our kids don't see us overly worried, so they know this isn't the end of the world, as it were, or anything of real significance. Sometimes, the more brusque, slightly less-empathetic comment can be more helpful ("Gosh, that doesn't sound very nice. Maybe you should play with someone else. She doesn't sound like a lot of fun.") than trying to talk it through...
... I will say this, in regard to 'standing up for themselves'-- she is only five. Of course, a nice birthday party would be awesome and she doesn't want to miss out. Thus, she may not be able to cognitively understand or relate to "standing up for yourself is better than...." simply because, for a five year old, NOTHING is better than exciting birthday parties with cake and ice cream. This is something most kids DO understand (which is why "you can't come to my birthday" is used-- heck, my son is in first grade and got that one leveled at him by another kid). So, she really isn't at a stage of emotional development to say "well, I would rather choose my dignity and self-respect than go to something everyone else is going to". That concept is WAY too far out for most little ones to grasp.
As I said, I agree with role-playing, puppet play, etc. as tools for getting comfortable speaking out. I also just want to encourage you that for many, many kids, this is a long journey, which may or may not come to a head in middle school and high school. The point of all this is to suggest, for now, easier options (go play with someone else) than focusing so much on the negative behaviors of others. It is good to let the teacher know, and then move on from this as much as you can. You may give her all the tools in the book for self-assertion, but until she can see/buy into WHY it's important, and put herself before 'what other kids think'... let's just say that she's going to have to develop into that. I do have empathy, G.-- it's what I'm waiting for mine to do as well. I just know that the more we talk about it, the more HE focuses on it and we all get pulled in. So, do try to keep it simple.