Grundrisse Book

Phew

The Grundrisse, being fragmentary, non-linear, unedited and never finished, is a whole lot more demanding than Capital, which in itself isn't a walk in the park either. It is filled to the brim with raw and not wholly assimilated information in the fields economic history and the discourse of political economy of Marx's contemporaries, oftentimes necessitating the reader to choose: am I actually interested in this? Am I in any way an audience for this, capable of understanding whatever's written? Much more than his other works, the Grundrisse is a place where Marx jotted down his developing thoughts, not an explanation of them. I personally found the first 300-or-so pages, roughly corresponding to "Part 1: On Money", altogether too abstract to permit understanding. The later half of the book (in "Part 2: On Capital") on agriculture, transportation, circulation and machinery, however, is hugely powerful and goes beyond what Marx would eventually write on the topics in Capital. As an added bonus, you get treated to prose as ominous and prophetic such as this:

Hence the highest development of productive power together with the greatest
expansion of existing wealth will coincide with depreciation of capital, degradation of the labourer, and a most straitened exhaustion of his vital powers. These contradictions lead to explosions, cataclysms, crises, in which by momentaneous suspension of labour and annihilation of a great portion of capital the latter is violently reduced to the point where it can go on. These contradictions, of course, lead to explosions, crises, in which momentary suspension of all labour and annihilation of a great part of the capital violently lead it back to the point where it is enabled [to go on] fully employing its productive powers without committing suicide. [21] Yet, these regularly recurring catastrophes lead to their repetition on a higher scale, and finally to its violent overthrow. There are moments in the developed movement of capital which delay this movement other than by crises; such as e.g. the constant devaluation of a part of the existing capital: the transformation of a great part of capital into fixed capital which does not serve as agency of direct production; unproductive waste of a great portion of capital etc.

Marx didn't edit this, so the reader will have to. If Capital leaves you with questions on the production and consequences of surplus value, turn to the Grundrisse. Selective recommendation.