The Communist Manifesto is very verbose and doesn’t serve as a good introductory work. I think the following suit that much better:
Why Socialism? - Albert Einstein
A basic overview of what is wrong with capitalism and why socialism is the solution. Very short.
The Principles of Communism - Friedrich Engels
Formatted as a set of questions and answers, answering basic questions like “what is communism?” and “What will this new social order [socialism] have to be like?”
The Triumph of Evil, Chapter 1 - Austin Murphy
Debunks commonly believed myths about 20th century socialist states.
Marxism-Leninism-Maoism Basic Course - CPI (M)
An in-depth overview of Marxist philosophy and theory, and the history of the socialist movement worldwide.
Critique of the Gotha Program, Section I - Karl Marx
Discusses the organisation of a socialist society.
If you can think of any others to add, feel free to leave a comment or reblog and add your own.
For communist anarchism Alexender Berkman’s: What Is Communist Anarchism is really good (although its a decent length)