For a basic intro, I would recommend "Principles of Communism" by Engels. It's a super quick read, and it's organized in a FAQ format.

Beyond that, it really depends on what in particular you're interested in. For economics, I can highly recommend "23 things they don't tell you about Capitalism" by Ha-joon Chang (although it's more about capitalism than socialism). For learning about the state, nothing compares to "The state and revolution" by Lenin.