Short answers, pulled from the story.
Rosa Luxemburg wrote the book between 1907 and 1914 while teaching economics at the party school of the Social Democratic Party of Germany. She completed the entire manuscript in four months during an intensive period of research.
Capitalism requires access to non-capitalist markets to survive according to Rosa Luxemburg's central thesis. This third market includes peasants, artisans, and colonial societies operating under modes of primitive communism or natural economy.
A society composed solely of capitalists and workers lacks an identifiable source of demand for capitalized surplus value within a closed system. Internal circulation cannot generate new money capital needed to realize growing surplus value without external sources from non-capitalist producers.
The book generated immediate fierce criticism from nearly all factions of the Marxist movement including Vladimir Lenin. Nikolai Bukharin argued her entire problem stemmed from misunderstanding capitalist motivation regarding worker consumption.
Rosa Luxemburg wrote a response titled The Accumulation Of Capital Or What The Epigones Have Made Of Marx's Theory: An Anti-Critique in 1915 while imprisoned during World War I. Published posthumously in 1921, this work defended her original thesis and sharpened arguments using what has been described as a macro-monetary perspective.