Reddit reviews The Philosophy of Marx

Since no one else has chimed in, here are my recommendations:

• The Development of the Monist View of History by Georgi Plekhanov (1895). This is still considered the classic on the subject, and is quite philosophical. Plekhanov was still a good Marxist at the time.
  
  
• Historical Materialism by Nikolai Bukharin (1921). This is a very underrated book that should be more famous. Bukharin was perhaps the most philosophical and theoretical of the Bolsheviks.
  
  
• The Philosophy of Marx by Etienne Balibar (1995). More modern, and a bit demanding. Balibar has some great insights and always poses interesting philosophical questions.
  
  
• Marx's Theory of History by William Shaw (1980). This book assumes you already are familiar with the subject. It is useful to read after all the others, as it clears up a lot of the conflicting interpretations and argues that Marx took a more deterministic approach than his followers.
  
  If I had to pick one book to start with, I'd go with Balibar.

Since the comments have already said to read Marx's primary material, I'll just suggest this as a secondary resource.