I also discovered the book Dialectics of the Abstract & the Concrete in Marx’s Capital, by Evald Ilyenkov.

I haven’t read it yet, but it seems like the book talks about how Marx develops his ideas from the abstract to the concrete. It might be useful for someone here.

Reading chapter 3 of The Capital. I really think this line is the best one:

[A]lthough money is nothing but the value-form of commodities, price ceases altogether to express value. Objects that in themselves are no commodities, such as conscience, honour, &c., are capable of being offered for sale by their holders, and of thus acquiring, through their price, the form of commodities.