Socialism: Utopian and Scientific

and

Value, Price and Profit, from which I've quoted most of the preface below:

In a partial sense the present volume is an epitome of the first volume of Capital. More than one of us have attempted to analyze and simplify that volume, with not too much success perhaps. In fact, a witty friend and commentator has suggested that what is now required is an explanation by Marx of our explanations of him. I am often asked what is the best succession of books for the student to acquire the fundamental principles of Socialism. The question is a difficult one to answer. But, by way of suggestion, one might say, first, Engels' Socialism, Scientific And Utopian, then the present work, the first volume of Capital, and the Student's Marx.

Edward Aveling

Socialism: Utopian and Scientific does a decent job of introducing you to how Marx is going to make his argument, and Value, Price and Profit does similar with showing what that argument will be.