The 27th and the 31st Infantry Regiments served in Siberia. The 339th Infantry Regiment served in North Russia. These were two separate zones of operation.
As far as my records show, the 27th and 339th were not amalgamated at any time. The 27th was a regular army regiment (i.e., part of the standing army), while the 339th was a National Guard regiment (part of the 85th Division).
The Doughboy Center (http://www.worldwar1.com/dbc/dbc2.htm)has good articles outlining both operations. The articles are in the First Army section.
As far as books are concerned, I recommend the following:
Halliday, E.M.: "When Hell Froze Over" (ISBN 0-7394-1595-6)
Miller, John: "The Day They Almost Bombed Moscow" (ISBN 0-689-11713-2)
Kindall, Sylvian G.: "American Soldiers in Siberia", Richard R. Smith, 1945
The first two are currently available; the third you should be available through used bookstores.
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