After a quick introduction, this document gives you the links to three letters as published at www.marxists.org.

    Marx lived from 1818 to1883, and Engels survived him by a dozen years (1820-1895). So, in the last part of the century, Engels became the interpreter of their joint and separate works. In these letters, he deals with the issue which many people have found the most difficult for "Marxism": the question of  "economic determinism". His position against a determinist reading is clear. (And of course, late in life, Marx said in frustration: "I am not a Marxist!")
    Please keep in mind that The German Ideology is from 1846: it is 45 years earlier than these letters, 2 years before the Manifesto, 21 years before the first volume of Marx's Capital, and before the innumerable events and debates of all those years. The German Ideology was written by a 27-year old Marx and 25-year-old Engels.

@Engels, Friedrich
        1890    Letter to J. Bloch in Konigsberg. (Text as first published in Der sozialistische Akademiker, #19, 1895.)

        1893    Letter to Franz Mehring.

        1894    Letter to Starkenberg. (I have also seen this letter published as "to W. Borgius". Don't ask me.)