I have to say that I didn't do too well with the Ethical Slut book, and I know other poly folks that haven't either.
One of the ways of thinking that I have found useful is to stop thinking of jealousy as the problem, but instead as a symptom. A symptom that you are not getting something that you feel you should bet getting.
This may be due to you having unrealistic expectations about what the relationship has to offer, but it might also be that your perfectly reasonable desires aren't being met.
Either way the best way to try to resolve this is to communicate with your loved one(s). Try to get to the bottom of what is going on, and see if you can reach some sort of agreement about how your needs can be met... or if they can't maybe you need to move on.