Knäcke (Knaecke) also called Knäckebrot (Knaeckebrot) refers to crisp or crispy bread. In Germany, they can be bought in plastic packs with at least 6 pieces plain or with assorted nuts, like white and black sesame seeds, poppy seeds, pine nuts or pumpkin nuts or mixed nuts, like the one in the picture below.
Knäcke is a healthy food and can be eaten by those who are dieting. especially the plain variety. They are always available in bakeshops and groceries all over Germany.
Personal Note: I enjoy mine with any kind of jam or marmalade on top and the one with pumpkin seeds and with a mixture of sesame and pine nuts
A list of books about the topic(s) Knäcke (Knaecke)