Reddit reviews Socialist Thought: A Documentary History

I highly recommend this book: Socialist Thought: A Documentary History.

You can go on further after and read the primary sources they discuss.

This lecture by Prof Wolff is good too.

Pre-Marx?

I don't know, but Albert Fried and Ronald Sanders, ed., Socialist Thought: A Documentary History discusses, and has excerpts from, these pre-Marx authors:

Early French Communism

• Morelly: Code of Nature
  
  
• Jean Jacques Rousseau: Discourse on Inequality
  
  
• "Gracchus" Babeuf: Letter to Dubois de Fosseux, Manifesto of the Equals, Analysis of the Doctrine of Babeuf, Excerpt from the Trial of Babeuf: Babeuf's Defense
  
  Utopian Socialism
  
  
• Claude Henri Saint-Simon: Letters from an Inhabitant of Geneva, The New Christianity
  
  
• The Saint-Simonians: Exposition of the Doctrine of Saint-Simon: First Session: On the Necessity of a New Social Doctrine
  
  
• Charles Fourier: Selected Writings ( Of the Role of the Passions, Of Education, The Phalanstery I and II, Attractive Labour )
  
  
• Robert Owen: An Address to the Inhabitants of New Lanark
  
  The Emergence of the Proletariat
  
  
• Chartism: "Crown and Anchor" Petition
  
  
• Louis Auguste Blanqui: The Man Who Makes the Soup Should Get to Eat It
  
  
• Pierre-Joseph Proudhon: What Is Property?
  
  
• Louis Blanc: The Organization of Labour
  
  Early German Socialism
  
  
• Johann Gottlieb Fichte: The Closed Commercial State
  
  
• Moses Hess: The Philosophy of the Act
  
  
  
  
  
  After that, there's a chapter on Marx/Engels, and then comes
  
  Anarchism: Bakunin, Kropotkin, Sorel (syndicalism), Cole (guild socialism),
  
  Revisionism: Lassalle, Webb (Fabian socialism), Jaurès, Millerand, Bernstein, Kautsky,
  
  Bolshevism: Plekhanov, Lenin, Kautsky (again), Trotsky, Stalin,
  
  Contemporary Socialism: Gramsci, Crosland,
  
  Newer Currents in Socialist Thought: Gorz, Eisenstein, West, Amin.
  
  But those aren't pre-Marx, and this took me way more time than I expected :)
  
  
  
  Some of these text can be downloaded as audiobooks here (1492-1789), here (1789-1865) or here (1865-).

Socialist Thought: A Documented History might be suitable for your purpose.