In the Wikipedia article it is written that fiefs were not necessarily lands:

However, not only land but anything of value could be held in fee, including governmental office, rights of exploitation such as hunting or fishing, monopolies in trade, and tax farms.

Can anybody recommend me some reading about this? A book or an article where these details are discussed. (In Wikipedia they don't give references on this. Actually, an article in an encyclopedia with some examples will also do.)