Facial expressions is one of my most favourite things to draw (maybe THE most xD)! This is probably gonna be weird, but I honestly think the best resource is your own face. Not kidding. Just get a mirror with a stand, put it beside you, and try to feel what your character is feeling in that scene. Try to emote it, and just reference what you see in the mirror. This sort of exercise is similar to life drawing, where you reference real life, and just implement it into your own style. You will get results you never expected to ever get.
I never owned a copy of a facial expressions book, though I'm sure it will offer some good aid, since there's distinctions between different people (though it's good not to get too obsessed with that >_>;; it almost took up my life) It would be good to just go back and forth from real life reference to books because even though book can give you great ideas, it can be limiting as well.
╭( ・ㅂ・)و ̑̑ I wish you the best!!