Author Upton ("The Jungle") or Lewis ("Main Street")
28A
Sort of
29A
Wipe
30A
Author Hilary (the "Wolf Hall" trilogy, which are brilliant, as is the TV adapatation starring Mark Rylance)
31A
Indian cultural center and capital of West Bengal
34A
Unassigned workspace, in modern lingo
35A
Addictive painkiller
36A
Birth-related
37A
Name meaning "born again"
38A
Canadian city famed (to me at least) for its bagels
41A
The "E" in ESL, for short
42A
Palindromic Vichy France leader Pierre ___, who was executed for collaboration with the Nazis (also the name of 38 Across's largest suburb)
43A
Like part of an AP language exam
45A
It might make you feel small
46A
Comedian Bargatze, for one
47A
Twee
49A
Harpsichord kin (go for Baroque!)
52A
"Not"
53A
Post field
54A
Shoe parts
55A
"I swear, kids ____" (also the title - just the words in the blank - of a beautiful Jackson Browne song, one of my favorites, written when he was only 16!)
56A
Exculpatory memos to file, for short
Verticales
1D
In Greek mythology, father of Agamemnon and Menelaus
2D
____ Coast (resort area southeast of Naples)
3D
Renowned New Yorker film critic who wrote "I Lost It at the Movies"
4D
Prefix for European or Chinese
5D
Rocksteady precursor
6D
Greek variant on an Italian staple, basically
7D
Completely behind
8D
Sole stud
9D
Leader who'd be stuck in mud if you moved the first letter to the end
10D
Your (Fr.)
11D
Swamp
12D
Swamps
13D
Manchego base
14D
Nautical directions
19D
Important nutrient in B vitamins
24D
One-time employer of Wernher von Braun
25D
Newborn, colloquially
27D
Barrel partner, commercially
28D
Bahrain neighbor
30D
Spot
31D
Any of Ted Williams, Casey Kasem, Neil Armstrong, or Ed McMahon, militarily