Any good sources for writing a scholarly document on Captain Marvel/Family, Plastic Man, or any other popular golden age public domain characters?
Try "American Comic Book Chronicles" 1940-1944, and "American Comic Book Chronicles" The '50s, published by Two Morrows. The late 1940s (1945-1949) hasn't (to my knowledge) been published yet. The volumes, mind you, give the reader a view of the history of The Comic Book Industry and its most influential publishing houses, and just some highlights of innovative new trends in composition of comic book lines, genres, types of characters, comic book formats, etc., what was produced and marketed at that time, and configuration and formats of books, etc.
For writing a scholarly document on individual Golden Age characters, your main sources should probably be simply reading many stories of your character in anthology books and their own titled series, by their publisher, covering all the different eras of those magazines' entire runs (e.g. enough representative stories from each period (including the most famous highlighted in golden Age comic book blogs, and books about individual publishers.
Some good sources are the following books on the following publishers:
(1) "Timely Confidential: When The Golden Age of Comics Was Young"
(2) "D.C. Comics, The Golden Age", "Fawcett Comics (1939-1953): A Journey Through The golden Age of Comic Books"
(3) "The Quality Companion"
(4) "E.C. Comics" (Rutgers University Press)
(5) "The Charlton Companion"
(6) "Will Eisner's The Spirit"
Here on CB+ read stories in the following comic book series:
Plastic Man:
Plastic Man and Police Comics (Quality Publishing)
Captain Marvel: (Fawcett Publications)
Whiz Comics, Wow Comics, America's Greatest Comics, Captain Marvel Adventures, Captain Marvel Storybook
Captain Marvel Jr.: Captain Marvel Jr. Comics, The Marvel Family (Fawcett Publications)
You should also look at our CB+ Compilations Section, for compilations of all the stories of lesser-known Golden Age superheroes (such as Blackhawk, The Blue Beetle, and others).
I hope all this helps you.