Before you read Das Kapital
You should read this book before you venture yourself in Das Kapital. Engels understood the man quite well and he gives you an opportunity to understand the scientific method applied by Marx in Das Kapital before you read those intense volumes. I read both together for my doctorate dissertation and Engels helped quite a lot to define my argumentative line in connection with Marx's theory. For that matter, read Marx in The Communit Manifesto to understand the classes struggle for economic power. If you wish to read Socialism: Utopian and Scientific but would like to have an insight from another author about economic historical facts, you should read Leo Huberman. He explains in a clear discourse the classes struggle for economic power in Man's Wordly Goods The Story of the Wealth of the Nations. Huberman unveils Marx's thoughts (and other philosophers of Economic theory) in his book. He published mainly for American readers, but was largely translated in many languages due to his writing style, I would guess.
