The grey economy is a part of the economy that is not taxed because it functions 'under the table'. It is illegal in the sense that it's a form of free riding, but it is not considered damaging by society so it is not illegal in a sense that stealing, counterfeiting or drug trafficking is.
The black economy (conducted on the black market) is part of the economy that is illegal not merely because it is not taxed but also because it is considered damaging by society.
I am looking for (intermediate level) literature that deals with estimation of the size and effect of these economies. It can also be about a technique used to estimate the size or effect of a specific sector of the black or grey economy, e.g. drug trade or ivory trade in a given area as long as the technique is general enough to be applicable with minor modifications elsewhere.