Hitler Postage Stamp

I found a book in my grandfather's attic after he died. It was a
German book of poetry for school children used during the 1930's.

Affixed to one of the pages was a postage stamp with Hitler on it. It
said "Deutsches Reich" on it and had the number 7 or 3 in the corners.
It had a profile of Hitler in the center.

Does anyone recognize this stamp and also are Nazi era postage stamps
common or rare?

Also does the stamp being affixed to a book lower its value?

Any help you can give I would appreciate.
David Kohlhoff