It could be convinient here to follow up on this thread on Ryazanov posted in September last year:
Quite recently, RGASPI (Russian State Archive of Social and Political History), located in the centre of Moscow (a house basically littered with enormous pictures of Marx, Engels and Lenin), published a new "guidebook" (putevoditel) to their collections; collections which include many works by Marx and Engels, history of socialism, workers' movements and political economy, and the work carried out at the MEL-institute, closed in 1991. What follows are some examples of what you can find here - the amount of material is huge.
The main file containing holdings on work at the MEL-institute is:
Fond 71, which holds almost 60.000 files.
Fond 374, which holds 66 files.
The main file containing the oeuvres by Marx and Engels:
Fond 1, holding more than 15.000 files, including original documents, books and letters.
Special attention should be drawn to Fond 1, opis 1, which preserves original documents from the period 1833-1895.
Another interesting file might be "Redaktsia 'Reinskoij Gazetij" (Rheinische Zeitung):
Fond 22, holding 66 files, covering the years 1840-1845.
Fond 20, holding 21 files, covering the years 1848-1849. (Neue Rheinische Zeitung)
Fond 430, containing 7 files, covering the years 1849-1851. (Neue Rheinische Zeitung. Politisch-Ökonomische Revue)
Marx' time in Brussels (equivalent to holdings in state archives of Belgium):
Fond 196, holding 93 files, covering the years 1845-1848.
Last but not least, I should mention the files on Ryazanov:
Fond 301, holding 61 files divided into two sections:
Opis 1, original documents and correspondence, 58 files, covering the years 1893-1931.
Opis 2, speeches, 4 files, covering the years 1917-1928.
These are just some fragments, if you are interested in exploring the holding in more detail I can unfortunately only recommend a book in Russian:
RGASPI - A Guide to the holdings of documents and museum files at RGASPI (Rosspen, Moskva, 2004).
If you are interested in following up on the tragic faith of many Russian marxist intellectuals, see for example:
"Menshevitskij protzes 1931 goda" books 1 and 2 (The Menshevik Process, 1931)
These two books contain the in extensio police interrogation protocols carried out by NKVD against noteable persons such as I.I. Rubin and D. Ryazanov. You know what they say, reality always beats fiction.