The article by James Field in First Empire is 7 pages long, and gives a lot of detail about the formation and origins of the units of the Russo-German Legion. Apart from Arndt, his other sources quoted were:

Beamish, N. Ludlow - Geschichte der Königlich Deutchen Legion, Vol 1-2 2nd edt, Berlin 1906.
Hubatsch, Walter (ed) - Karl Heinrich Freidrich vom Stein, Brief und amtliche Schriften, Vol 3-4, Stuttgart 1963
Niedersächsiches Staatsarchiv in Oldenburg, Bestand 63, Akten I-XXXIV
Schirmer, Friedrich - Nec aspera terrent, Vol 2, Hildesheim 1937
Quistorp, Barthold v - Die Kaiserlich Russich-Deutsches Legion, Berlin 1860
Quistorp, Barthold v - Geschichte der Nordarmee im jahre 1813, Vol 1-3, Berlin 1894
Sichart, U und R v - Geschichte der Königlich-Hannoverschen Armee, Vol 5, Hannover 1898
Venzky, Gabriele - Die Russisch-Deutsche Legion in den Jaren 1811 - 1815, Wiesbaden 1966
Wersebe, Wilhelm v - Geschichte der hannoverschen Armee, Hannover 1928

You might be able to find the Graf von Munster letter to Stein in one of these sources. I would guess most probably in Hubatsch or the Niedersächsiches Staatsarchiv.

I have Beamish (the English version) and photocopies of some extracts from both Schirmer and Sichart, but no access to the other books.

There is a lot of fascinating detail in the article, including established strengths, unit commanders, sources of soldiers for each of the units etc. There is also the interesting statement that the Prussians expected Gneisenau to be the RDL C-in-C but since Britain was financing it they insisted on von Wallmoden-Gimborn (who was a Hannoverian officer at that time in Austrian service). This offended Gneisenau, who wrote insinuating that another reason was that Stein and Wallmoden were brothers-in-law. James Field speculates that this incident may have contributed to Gneisenau's strong mistrust of the British during the Waterloo campaign.

Rod