Here's one of my most recent primer lists:
- Why Socialism? by Albert Einstein
- Principles of Communism - Engels
- Communist Manifesto - Engels & Marx
- Elementary Principles of Philosophy by Georges Politzer
- Wage Labour and Capital - Marx
- Value Price and Profit - Marx
- The Three Sources and Three Component Parts of Marxism - Lenin
- Reform or Revolution - Luxemburg
- What Is To Be Done - Lenin
- The State and Revolution - Lenin
- Inventing Reality - Parenti
- Blackshirts and Reds - Parenti
Not saying it's bullet proof list but this should cover a lot of what socialism actually is. Like Femboy said, I think starting with Einstein is a baller move. I always tell people to read Principles of Communism before the Manifesto because it's super easy to read and now you have the foundation to go off of once you hit Marx(which can be pretty dense at times). Elementary Principles of Philosophy is sort of a pallet cleanser but it's a great intro to dialectics. It will get them to start thinking about materialism as they read more. Wage Labour is the one I would recommend if you can only read one. It's probably my favorite work by Marx as it solidified my own views on capitalism as a wage slave when I was first starting theory. The rest just sort of falls in place after that.
I think I would also put Wretched of the Earth by Fanon as an honorable mention. I'm currently working through it but it really hits home the relationship between the owner class and the proletariat but it does it from the lens of Black untra-exploited people in the global south. I saw someone suggesting some theory on reddit a few days ago and when pressed on why not this book, he said it lacks dialectics. But I disagree whole-heartedly. This book is pure material and historical dialectics.