A long essay or short book tackling Marx's views on human nature, and disputing the view that Marx believed there was no immutable human nature.
This is somewhat interesting - although it is in essence a polemic against a structuralist view of Marx which has I suspect largely fallen out of favour since this book was written. Geras' view is that there were a number of ways in which Marx did support the idea of there being certain core traits that marked out what it is to be human, above all the need to produce and re-produce. This is in general a convincing refutation, built largely on top of a close reading of the sixth thesis on Feuerbach.
However it felt to me that there was something missing. In particular the common view is that human nature is indelibly connected to capitalism, that it is in our nature to "truck, barter, and exchange". Geras' argument is pointed almost entirely to other Marxists, and really doesn't engage much with this view that 'human nature' is in fact a barrier to moving beyond capitalism. He does argue that accepting a common human nature does not determine exactly what that nature is or what limits it places on future development. But clearly if we accept the point then it must place some limits on future development and I am not fully convinced that this is what Marx was getting at.
Either way the argument fits into the category of debates around "what Marx really meant" which is in general an unhelpful distraction from making profitable use of his analysis to understand modern capitalism and how that should influence practical politics in the here-and-now.