The book is by William MacAskill, founder of 80000 Hours and Giving What We Can. Excerpt:
Effective altruism takes up the spirit of Singer’s argument but shields us from the full blast of its conclusion; moral indictment is transformed into an empowering investment opportunity...
Either effective altruism, like utilitarianism, demands that we do the most good possible, or it asks merely that we try to make things better. The first thought is genuinely radical, requiring us to overhaul our daily lives in ways unimaginable to most...The second thought – that we try to make things better – is shared by every plausible moral system and every decent person. If effective altruism is simply in the business of getting us to be more effective when we try to help others, then it’s hard to object to it. But in that case it’s also hard to see what it’s offering in the way of fresh moral insight, still less how it could be the last social movement we’ll ever need.
That's the question, what is the base rate of the options you are likely to notice. If visible causes come in equivalent pairs, one with harm in it and another without, all other traits similar, that would be true. Similarly if pure benefit causes tend to be stronger. But it could be the case that best pure benefit causes have less positive impact than best mixed benefit causes.
How does your taking disagreements seriously (what do you mean by that?) inform the question of whether most real (or just contentious?) causes involve actual harm as well as benefit? (Do you mean to characterize your use of the term "disagreement", which causes you point to as involving disagreement? For example, global warming could be said to involve no disagreement that's to be taken seriously.)
Yes, it could be the case that the best pure benefit causes have less positive impact than the best mixed benefit causes. But I have no special reason to believe this is the case. If benefit of the doubt is going to go to one side without argument, I would put it on the side of pure benefit causes, since they don't have the additional negative factor.
By taking disagreements seriously, I mean that I think that if someone disagrees with me, there is a good chance that there is something right about what he is saying, and especially in issues of policy (i.e. I don't expect people to advocate policies that are 100% bad, with extremely rare exceptions.)
That global warming is happening, and that human beings are a substantial part of the cause, is certainly true. This isn't an issue of policy in itself, and I don't take disagreement about it very seriously in comparison to most disagreements. However, there still may be some truth in the position of people who disagree, e.g. there is a good chance that the effects will end up being not as bad as generally predicted. A broad outside view also suggests this, as for example in previous environmental issues such as the Kuwait oil fires, or the Y2K computer issue, and so on.
In any case the kind of disagreement I was talking about was about policy, and as I said I don't generally expect people other than Hitler to advocate purely evil policies. Restricting carbon emissions, for example, may be a benefit overall, but it has harmful effects as well, and that is precisely the reason why some people would oppose it.