Some of these I've scanned myself, most of the others were scanned by people I know.
A Soviet history of Europe and the Americas covering 1640-1870: https://archive.org/details/ModernHistory16401870
A Soviet history of the world covering 1917-1945: https://archive.org/details/AContemporaryWorldHistory19171945
A Soviet history of the world covering 1917-1990: Volume I (1917-1945), Volume II (1946-1990)
USSR:
* https://archive.org/details/HistoryUSSREraSocialism (covers Soviet history from 1917-1980)
* https://archive.org/details/TheUSSRAShortHistory (covers Soviet history from 1917-1975)
China:
* https://archive.org/details/ModernHistoryChina (covers the 17th century up to 1918)
* https://archive.org/details/ConciseHistoryCPC (covers the period preceding the formation of the CPC up to the start of the 90s)
* https://archive.org/details/in.ernet.dli.2015.98145 (covers 1919-1956)
* https://archive.org/details/YearsOfTrialTurmoilTriumph (covers 1949-1980s)
United States: http://uselectionatlas.org/FORUM/index.php?topic=202462.0 (all sorts of Marxist works on US history here)
Poland:
* https://archive.org/details/TopolskiPoland1986 (entire history of Poland up to the 1970s)
* https://archive.org/details/ClassStruggleInSocialistPoland (covers Poland from the early 20th century to the early 80s, with a focus on the workers' movement followed by an analysis of the Polish state and economy after 1945 and rise of Solidarity)
Hungary: https://archive.org/details/HistoryHungary
Germany:
* https://archive.org/details/TheLessonOfGermanyGuideHistory (history up to the end of WWII)
* https://archive.org/details/GDRHistoricalOutline (history of the GDR going up to 1979)
Macedonia: https://archive.org/details/HistoryMacedonianPeople
Bulgaria: https://archive.org/details/EvansShortHistoryBulgaria
Mongolia: https://archive.org/details/HistoryOfTheMPR (entire history of Mongolia up to 1970 or so)
Central Asia: https://archive.org/details/CentralAsiaInModernTimes (from the early 19th century onward)
Korea: https://archive.org/details/ModernHistoryOfKorea (covers 1919-1970s)
Vietnam: https://archive.org/details/OutlineHistoryOfTheVietnamWorkersParty (covers 1930-1975)
Cambodia:
* https://archive.org/details/ChinaCambodiaVietnamTriangle (focused on the rise and regime of Pol Pot)
* https://archive.org/details/KampucheaTheRevolutionRescued (ditto except also has some stuff on Cambodia in the 80s)
Albania:
* https://archive.org/details/TheHistoryOfAlbaniaABriefSurvey (whole history up to the 1950s)
* https://archive.org/details/TheHistoryOfTheSocialistConstructionOfAlbania (covers 1944-1975)
Afghanistan: https://archive.org/details/AHistoryOfAfghanistan
African countries (as well as Africa itself): https://www.revleft.space/vb/threads/194824-Soviet-and-other-works-on-Africa-(PDFs)
A Soviet history of Europe and the Americas covering 1640-1870: https://archive.org/details/ModernHistory16401870
A Soviet history of the world covering 1917-1945: https://archive.org/details/AContemporaryWorldHistory19171945
A Soviet history of the world covering 1917-1990: Volume I (1917-1945), Volume II (1946-1990)
USSR:
* https://archive.org/details/HistoryUSSREraSocialism (covers Soviet history from 1917-1980)
* https://archive.org/details/TheUSSRAShortHistory (covers Soviet history from 1917-1975)
China:
* https://archive.org/details/ModernHistoryChina (covers the 17th century up to 1918)
* https://archive.org/details/ConciseHistoryCPC (covers the period preceding the formation of the CPC up to the start of the 90s)
* https://archive.org/details/in.ernet.dli.2015.98145 (covers 1919-1956)
* https://archive.org/details/YearsOfTrialTurmoilTriumph (covers 1949-1980s)
United States: http://uselectionatlas.org/FORUM/index.php?topic=202462.0 (all sorts of Marxist works on US history here)
Poland:
* https://archive.org/details/TopolskiPoland1986 (entire history of Poland up to the 1970s)
* https://archive.org/details/ClassStruggleInSocialistPoland (covers Poland from the early 20th century to the early 80s, with a focus on the workers' movement followed by an analysis of the Polish state and economy after 1945 and rise of Solidarity)
Hungary: https://archive.org/details/HistoryHungary
Germany:
* https://archive.org/details/TheLessonOfGermanyGuideHistory (history up to the end of WWII)
* https://archive.org/details/GDRHistoricalOutline (history of the GDR going up to 1979)
Macedonia: https://archive.org/details/HistoryMacedonianPeople
Bulgaria: https://archive.org/details/EvansShortHistoryBulgaria
Mongolia: https://archive.org/details/HistoryOfTheMPR (entire history of Mongolia up to 1970 or so)
Central Asia: https://archive.org/details/CentralAsiaInModernTimes (from the early 19th century onward)
Korea: https://archive.org/details/ModernHistoryOfKorea (covers 1919-1970s)
Vietnam: https://archive.org/details/OutlineHistoryOfTheVietnamWorkersParty (covers 1930-1975)
Cambodia:
* https://archive.org/details/ChinaCambodiaVietnamTriangle (focused on the rise and regime of Pol Pot)
* https://archive.org/details/KampucheaTheRevolutionRescued (ditto except also has some stuff on Cambodia in the 80s)
Albania:
* https://archive.org/details/TheHistoryOfAlbaniaABriefSurvey (whole history up to the 1950s)
* https://archive.org/details/TheHistoryOfTheSocialistConstructionOfAlbania (covers 1944-1975)
Afghanistan: https://archive.org/details/AHistoryOfAfghanistan
African countries (as well as Africa itself): https://www.revleft.space/vb/threads/194824-Soviet-and-other-works-on-Africa-(PDFs)
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