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.)

EDIT 18.08.2020. From people's reaction I see that there is no common agreement about the meaning of the term "fief", so perhaps it will be reasonable to clarify how I understand it. From what I read about this I got an impression (people can correct me) that

a fief is a source of income (not necessarily, land, this as well can be a permission to trade somewhere, or a monopoly on the production of something, etc.) with the following three distinguishing features:

• this source of income is granted to a person (called a "vassal" in Europe, but of course the concrete words are not important), by another person (called a "seigneur" in Europe) under a certain treaty (which is a part of the tradition),
• in this treaty it is supposed that (in gratitude for this gift) the vassal is obliged to provide certain services to the seigneur (again, the content of these services can be very different, it can be regular payments to the seigneur, or military service, or service at the seigneur's court, or, as far as I understand, even something unclarified), and
• the seigneur has a formal right to cancel this treaty if he decides that the vassal does not fulfill his obligations (of course, the possibility to do this depends very strongly on the tradition and on the concrete situation, but in this culture everyone recognizes the formal right of the seigneur to cancel the treaty).

My question is where all these details are described? I see many books about fiefs understood as lands, but only fragmentary mentionings of fiefs understood as other sources of income. I cannot find a text where these things are discussed in detail (or even simply explicitly formulated as I did here). Does anybody know such a text?