This is my first post, so. Hi.

Question: Has anyone found a book or website that discusses how to recognize early on if a person is going to be passive aggressive?
I have a pretty extensive history of trauma, and have found that the most damaging people in my life have been passive aggressive "friends". The kind of folks who never discuss feelings or boundaries, seem sweet at first, then build up resentment. They then become petty, avoidant, and incredibly hurtful.
I'm not drawn aggressive types, so I'm not involved with them socially. Most of what I've seen is about narcissists. The people I'm referring to are not narcissists. Any suggestions? Thanks.
I don't know of any books or websites off the top of my head, but reading your post reminded me of a former friend of mine. That friend used to be best friends with an old friend of mine, and they broke up because of the former friend's passive-aggressiveness... it was really messy. It also made me realize that I've also been guilty of being passive-aggressive myself (I already realized I'm a passive person and want to work on enforcing my boundaries and being honest when I'm unhappy with something instead of going along with it...), unfortunately. Now that I think about it, I've been surrounded by (or maybe I surrounded myself with) people who are passive-aggressive... parents, siblings, friends, classmates... I wanted to say thanks for your post, because I probably wouldn't have realized this until much later.
Unfortunately, I don't know how to detect a passive-aggressive person early on, because like you said, they seem very nice, but once you're deeper into the relationship, it's too late. I'm not sure if it matches up with your experience, but it's like their true colours don't show until you've established a relationship with them? Like, at the beginning they don't know you too well so they aren't as comfortable at expressing any dissatisfaction they have towards you, but as the relationship goes on, they get more and more comfortable being angry at you, but they still do it indirectly, so it's like you feel like you're just kidding yourself. And they feel entitled to their anger/to you having to pander to them because the relationship has progressed and therefore it means their feelings should be important/known to you, but the other party isn't aware because the passive-aggressive person keeps hiding their feelings. It's like you can't tell because they may say nice things/act nice on the outside, but on the inside, their feelings/thoughts don't match. For all you know, they are completely fine with you or with what's happening.
I guess I would just try to trust my gut instinct more whenever I feel like someone feels negative when they're acting like they're fine (which is hypocritical of me, because I tend to dismiss my gut feelings because I have a hard time trusting myself/I think I'm just being unreasonable).
Passive aggressiveness reflects more on the person than you. It shows that they aren't willing to state their feelings and still expect you to understand what they feel. I have learned that the friends that are really worth it are the ones that make you feel good. Of course, it's a lot harder when you have one or very little friends to begin with. At some point it feels better to have toxic friends than no one sometimes.
I like what waitingforrest wrote. Especially with the statement that "passive aggressiveness reflects more on the person than you." Though when I feel hurt by something someone did, I can recognize when it wasn't meant to intentionally hurt me, and I don't expect people to understand me anyway when I don't say anything (I acknowledge that it's because I don't say anything, I just fear confrontation, which I need to learn to overcome), or expect people to spend a bunch of time figuring out what they did and then get mad that it took so long when they finally do. I'm glad I haven't had to be on the end of a passive-aggressive person's anger most times, somehow I'm deemed nice and normal enough to be on people's good side (but I guess I'm not similar enough to them to ever have deeper relationships). But now when I look back, these people have been passive-aggressive towards others.
Maybe because of trauma, maybe because they're jealous of you, maybe because they want to be polite, maybe they're shy, maybe because they don't want to hurt your feelings, maybe because they are insecure about themselves/their thoughts, maybe because of trust issues... whatever the reason, they are holding themselves back from being upfront with you about their true feelings. But it doesn't excuse them from hurting others, and it doesn't mean they should get sympathy from those they hurt.
I think I'm talking too much about myself now, but I tried finding some links that could help answer your question. Hopefully it helps, though you may have already seen these. And I feel like a lot of these just state the behaviours of a passive-aggressive person, which you're probably already really familiar with due to your history with passive-aggressive people: