Readers familiar with the biography of Karl Marx will recognise some of Liebknecht's book as the major sources. Liebknecht's account of a pub crawl along Tottenham Court road in which Liebknecht, Marx and their (then) friend Edgar Bauer has become a oft-told story among left-wingers who gleefully recount how the group got into a near fight after challenging some "old-fellows" English patriotism and then fled the police after breaking numerous street lamps at 2am. The book is also the likely source for the accounts of Marx's love of chess and cheap cigars.
It's a melancholy book. Wilhelm Liebknecht is writing his memories, together with the recollections of Eleanor Marx, towards the end of his life and the final section - when he returns to the London of his youth to find the places that he, Marx and the wider circle of exiles argued, debated and laughed - is tinged with real sadness. Marx was clearly a towering political figure for Liebknecht, but also a close friend - the description of his first meeting with Marx and Engels as they cross-examine him over beer and food gives an idea of how Marx would allow people into his inner circle, but only if they could demonstrate their political principles. Once in that circle however, Marx and his family would gladly give everything they could.
The book is not easily available. But it is online at the MIA while it is not a starting point to understand Marx's ideas - it is the basis to understand him as a person.
Mehring - Karl Marx: The Story of his Life
Gonzalez - A Rebel's Guide to Karl Marx
Marx - Capital Volume I
Löwy - The Theory of Revolution in the Young Marx