Has someone written a monograph on ethics related to social darwinism and darwinism?
If you want an overview the Internet Encyclopedia of Philosophy has a page on evolutionary ethics that provides an excellent start. However, if you are only interested specifically in social darwinism (and not other interactions between evolutionary concepts and morality) then you should go straight to the page on Herbert Spencer.
Herbert Spencer is a 19th century English theorist who is pretty much synonymous with the subject of social darwinism. Almost ironic given his reputation, his moral theory also strongly argues for the importance of charity and helping the weak. However, since society is a struggle for survival and resources against other groups, people should withhold resources from inferior rival groups.
For more you can search Google Scholar for "social darwinism" to get plenty of published papers. The pages on Wikipedia and IEP also have references that would be worthwhile to follow up on.
Some Specific Sources
One of Herbert Spencer's earliest works in this vein is an essay called Progress: Its Law and Cause. The entire work was published in a scholarly journal, but you can find an excerpt here.
Although I haven't read it myself, Social Darwinism in American Thought seems useful. It's a scholarly work about social darwinism, rather than a primary text. It's been cited in numerous academic articles, so there's some legitimacy to it.