These are rather expensive but highly esteemed.
The best have thick, small (1-2/1 diameter) caps, light brown in color with prominent white cracks and a subtle fragrance; serve these whole. For boiling in soup or slicing to stir-fry use cheaper, larger, darker ones. In any case buy only those very dry, with no signs of insect damage, particularly on the underside of the cap, and with a good fragrance.
To use, remove tough stems and save for soup. Soak caps in water at room temperature for 30 minutes to several hours; use soaking water in cooking later.