It seems like it''s easy to find information about the various schools of writing up through modernism and post-modernism, but from then to the present it''s tough to find things neatly grouped and labelled. Can anyone enlighten us?

mmmh, I dunno, maybe it''s because you are researching american litterature ??? Did you try french litterature for instance. I know that in school we learn stuff from the 16th century up to the 19th... I dunno anything about the 20th for the simple reason that we spend most time studying the elders.
For instance, I remember vividly all the romantism and realism (or whatever the technical word is) of 19th century writers like Flaubert, Balzac, or Hugo (you probably know "Les Miserables").
Baudelaire was probably the first goth by modern criteria...
18th century is (in my memory) mainly interested in Philosophy.
17th century, we studied mostly theatre authors. Moliere, Racine, Corneille, et al. Either they base their stuff on classic greek tragedy (Racine, if I am correct), or on the emerging Comedia del Arte (italian comedy) writing comedy that carries a message for the initiated. A bit like the tales of La Fontaine, describing the adventures of animals, were only satire of the society under Louis the XIVth, were animals represented existing characters ...

youpla :-P

And please correct me, I am writing this from the top of my head, and my memory is very vague ... sometimes