The Black Tanker – January 1, 1941. Howard Pease.
This was the first novel about the sea and life on merchant ships I read a a kid in the 7th grade.
The price of the *Black Tanker * seems a bit off the charts as it and his other books have never been digitized (I assume).
There was also C.S. Forrester’s Hornblower, Guy Gilpatric’s Glenncanon series, and Douglas Reeman’s (aka Alexander Kent) Bolitho series. And now even more period naval novels than ever have sprouted up, but few if any about merchant ships.