Tristram Hunt, "Engels: The Man Marx Called General" - A recommended biography to learn about the life and thought background of the genius who supported Marx!
This biography does not take the position of overly glorifying or attacking Marx or Engels. It distances itself from such excessive ideological bias and is based solely on historical facts.
And this book showed me how much Engels had influenced Marx's writings.
It was such a great biography that I think you will learn more about Marx by reading this book than by reading a biography or commentary of Marx. I would highly recommend reading this book in addition to the biography of Marx.