Step 1: The question asks about the characteristic tone and atmosphere of the poet Walter de la Mare's work, as exemplified by the poem "Martha."
Step 2: The poem "Martha" describes a storyteller whose tales are filled with fairies, gnomes, and enchanting, tranquil imagery.
Step 3: The overall effect of such storytelling is to create a world that is not ordinary. It is a world of magic, dreams, and the unknown. This creates a distinct atmosphere of 'mystery' and wonder, drawing the listener or reader away from the everyday into a fantastical realm.