I’ve been reading The Power of a Positive No: Save the Deal Save the Relationship – and Still Say No by William Ury, the author of the earlier bestseller, Getting to Yes.
Ury is an experienced mediator, at levels from the corporate to the national, and he is a co-founder of the Harvard Law School Negotiation Programme. I found his writing very helpful. just as I do the related Disagreeing Well initiative at University College London, where I was an undergraduate.