You will find under this heading, of course, covers on sea posts. On the one hand a lot about the transatlantic service (North Atlantic and only HAPAG - So Hamburg/Cuxhaven to New York). And the North Atlantic services of the USA - as well as all the sea mail services of the USA. All of them from the beginning (the first one started in 1891) until the end in 1939/41. After WWII, these maritime services, just with on-board post office of the corresponding postal administration, were not resumed.

Literature on the German sea posts in the North Atlantic (there was actually only the "Deutsch Amerikanische Seepost"/"US German Sea Post" - carried out together by HAPAG, Norddeutscher Lloyd and later also the United States Line) is abundant, as on most other sea posts (to the German colonies etc.).
There are only two publications on US Seaposts (there were also much fewer than German ones) - Once by Cockrill four volumes, two each on German Seapost cancels and two on US Seapost cancels. Year of publication is unknown to me and cannot be determined. And from Hosking only about the US Seaposts from 2008.

Associations dealing with maritime mail seem to exist only two worldwide. On the one hand in Germany (main occupation German sea mail) the "ArGe Schiffspost im BDPh". And in England (the book by Hosking comes from there - but main occupation is England and her former colonies - and also railroad mail) the "TPO and Seapost Society". Then on a volume of Cockrill still one in the USA is indicated, the "Maritime Philatelic Society", which I cannot find in the Internet however - to it, after inquiry with the APS, no society (i.e. of the members) on this subject is known there in the USA.

I then found another association in the USA. The "Universal Ship Cancellation Society". As far as I read on their homepage, they probably only deal with the US Navy and its ships and corresponding cancellations.
1857 – Hamburg – Aachen – Liverpool – Boston – New York

A "double-rate paid letter" written in Hamburg on April 18, 1857, postmarked at the city post office on April 22, 1857. Note at the top on the letter "Via Liverpool p Steamer of April 25". The letter was sent to the Exchange Office for the Prussian Closed Mail in Aachen. There it received the postmark "Aachen/(date)/PAID 50 cts." with the date April 23. From here it went in a closed Mail bag via Belgium to Liverpool. On April 25, 1857, the Cunard Line ship "Europa" sailed and reached Boston on May 7.

In Boston, the bag was opened on the same day and received three stamps. The first - "BOSTON/(date)/BR. PKT./ PAID", the second. stamp with an incorrect "45"(cents) as prepayment, and then the third correctly with "60"(cents) as prepayment and therefore paid in full. Since the business letter contained an invoice for a shipment of goods, it went to customs where a postmark dated 26 May 1857 was struck.

Translated from „Understanding Transatlantic Mail Vol I“)
Copyright Jürgen Kuseler 2023