I don't know anything about the Shadow in the Sands, but you can't go wrong reading Riddle of the Sands - a brilliant book that's been popular over a very long period of time.
.Would it be better to read "The Riddle of the Sands" first? Peter.
In a word - yes!
"The Shadow in the Sands" by Sam Llewellyn is an account of the cruise of the yacht GLORIA in the Frisian Islands in the April of 1903, and the conclusion of events described by Erskine Childers in his narrative The Riddle of the Sands
The book continues the celebrated story of maritime intrigue, treachery, and adventure begun in Erskine Childers' timeless thriller.