For that she really lost it and has since told her siblings she's going no contact with me.
Words are cheap and BPDs say all kinds of things when they're dysregulated. Don't think too much into this. She will contact you or she won't.
You have your priorities in order with the graduation approaching. Don't let your older child ruin that.
There's nothing wrong with buying a book. I can see the over-reaction because the entire family saw that you bought a mental health book. Your BPD kid took it as a personal attack. The whole reason you bought the book, however, is because she takes a lot of things as personal attacks and wants to argue over it.
If she confronts you directly, be honest- you bought the book to better understand her and you're hoping to avoid the same types of conflicts in the future. But don't defend, don't argue, because that's what she wants...that's what feeds her mental illness. You have to get away from that sort of thing for the relationship to change.
Honestly, no contact may be the best thing right now. Read your book in peace and be better equipped for the next vacation.
One last thing- she doesn't want to go no contact...she wants you to apologize and beg her to forgive you. Again, that's the mental illness aspect of this. I would advise you to resist that temptation to say whatever she wants just to "fix things", because it's not actually fixing anything. It's being manipulated and it makes the situation even worse over time.