Richard Frajola, on his Philamercury Board, has posted a link to an Exhibit/Monograph which is worthy of review by all collectors, even those who do not specifically collect postal history.
Titled: How They Carried the Mail in America: 1850-1870
, and prepared by George Tyson, it does not follow the traditional postal history format, as it consists of only 61 covers described within 122 pages, but each item was selected to illustrate a specific purpose and each is described with considerable research to provide the proper historical background.
It can be viewed here:https://www.rfrajola.com/Tyson2019/GT2019.pdf
and downloaded if desired.