Here is a book offered by the USPS, the historic postal operator in the United States. Free of charge because in this country, the work of the federal government payed by the citizens' taxes must be at available to the largest possible peoples (that's why you find so much NASA space pictures than ESA...).
Freely available means for example that the whole text is readable on the USPS website.
I found this opportunity thank to The Virtual Stamp Club, a English-written philatelic forum. The link will conduct you directly to the postal adress to contact. Apparently, to my own surprise, this service accepts to honor demands from foreigners. Thank you very much, I won't waste this opportunity to learn and teach US postal history.