Start here:

• Replaceability (Paul Christiano, 2013)
• Four flavors of time discounting I endorse, and one I do not (Paul Christiano, 2013)
• Giving now vs. later: a summary (Julia Wise, 2013). This also contains many links to posts written on this topic before 2013; below I've only listed further reading which isn't already mentioned in Julia's post.

Further reading:

• Which Ethical Careers Make a Difference?: The Replaceability Issue in the Ethics of Career Choice (Ben Todd, 2012)
• The replaceability effect: working in unethical industries (Seb Farquhar, 2012)
• Economics of small changes (Paul Christiano, 2013)
• It's harder to favor a specific cause in more efficient charitable markets (Carl Shulman, 2014)
• Philosophers vs economists on discounting (Carl Shulman, 2012)
• Parable of the multiplier hole (Robin Hanson, 2010)
• Further than Africa (Robin Hanson, 2010)
• Risk aversion and investment (for altruists) (Paul Christiano, 2013)
• RPTP is a strong reason to consider giving later (Phil Trammell, 2018)
• Discounting for public policy: a survey (Hilary Greaves)