Dried shiitake mushroom is an essential ingredient for temple cuisine, also soaking liquid is also used as stock. The component of its umami is called guanylic acid. Its taste and flavour is chemically increased by drying as well as its nutrition value of vitamin D2.
It is said that umami is maximised when dried shiitake is soaked in water longer (over 24 hours), but if there is not enough time, you can soak it in warm water with a little sugar to rehydrate faster.
There are several types of dried shiitake including donko (thick and closed cap), koushin (thin and open cap) and hana-donko (donko with crack patterns).