Marx’s theory of the “value-form of the commodity” was developed over four different published texts. I have linked to them below in chronological order. It’s very interesting to read them in succession in terms of how Marx tried to make it easier for the reader. There’s something to be gained from using each text and as such a later version does not necessarily supersede the former. My preference is to usually use the third version (the appendix to the first German edition of Capital) because it is structured in a very pedagogical manner, unlike the first version which is especially dense reading.
Always keep in mind Marx’s own reflections on this body of work, too:
“Every beginning is difficult, holds in all sciences. To understand the first chapter, especially the section that contains the analysis of commodities, will, therefore, present the greatest difficulty. That which concerns more especially the analysis of the substance of value and the magnitude of value, I have, as much as it was possible, popularised. The value-form, whose fully developed shape is the money-form, is very elementary and simple. Nevertheless, the human mind has for more than 2,000 years sought in vain to get to the bottom of it all, whilst on the other hand, to the successful analysis of much more composite and complex forms, there has been at least an approximation. Why? Because the body, as an organic whole, is more easy of study than are the cells of that body. In the analysis of economic forms, moreover, neither microscopes nor chemical reagents are of use. The force of abstraction must replace both. But in bourgeois society, the commodity-form of the product of labour — or value-form of the commodity — is the economic cell-form. To the superficial observer, the analysis of these forms seems to turn upon minutiae. It does in fact deal with minutiae, but they are of the same order as those dealt with in microscopic anatomy.”
It would seem that the anatomy of capitalism is laid out in these four texts…