I am interested in learning about the histories of modern medical specialties but cannot seem to find much scholarly work.
A lot of work has been done in psychiatry. This field seems to be the rare exception.
Neurosurgery is somewhat different because it is considered-- correctly or not-- to have been mostly founded by an individual, Cushing.
I have also found some "histories" of diagnostic radiology online, mainly chronicling the advent of each modern imaging modality. These timelines usually stop with adoption of PACS a few decades ago.
Apart from some PubMed articles, I can find little on the other specialties. For example Internal Medicine and Family Medicine (and why these two specialties are separate in the first place), Emergency Medicine, Neurology, General Surgery, and Obstetrics/Gynecology.
Can someone point me to some resources?
You could try this bibliography from Duke University Press. I see a book on neuro-oncology (albeit from 2006, and not from a Press I recognise), and pediatrics. I would also suggest looking at the websites of public agencies, where bureaucratic histories may be more available.