I prefer Conrad's short pieces to his long pieces. This, one of his very best short pieces, is similar in theme to his best long piece, Lord Jim. In this late, short work, Conrad seemed to leave behind his cacophony of words that, for example, Nostromo and the The Secret Agent suffer from. The writing is simpler, less wordy and more evocative than much of Conrad's. You can really breeze through The Shadow Line, which is something that can be said of very few Conrad works.
The story, too, is exciting and poignant. A young man is appointed to his first command and has to take his ship the 800 miles from Bangkok to Singapore, but the voyage is 'cursed' by plague and calm weather. There are hints of the supernatural, and an underlying metaphor of war (WW1 was raging while the book was being written).
The Shadow Line's opening 'youth', newly appointed to his first command, may be an old man's idea of youth - brash, rude, arrogant and ignorant (see Murakami's Kafka on the shore for another old man's, quite different, idea of youth), but it's an attractive, charismatic idea nonetheless. The character's development, through 'the shadow line', into maturity, is completely convincing.