It depends on what aspect of theory you're interested in.
For introductory material, I can highly recommend Blackshirts and Reds by Parenti. It's well written, and gives a good overview of socialism in a modern context.
I thought these books discussing socialism in the context of China were pretty good as well
- The East is Still Red https://redletterspp.com/products/the-east-is-still-red
- People's China at 75 – The Flag Stays Red https://redletterspp.com/products/peoples-china-at-75
- China's Great Road https://redletterspp.com/products/chinas-great-road
For an intro on the modern economic theory aspect, Geopolitical Economy Hour with Radhika Desai and Michael Hudson is pretty good
https://www.youtube.com/watch?v=jL1ObLixT8M&list=PLDAi0NdlN8hMl9DkPLikDDGccibhYHnDP
It’s telling how quickly liberals leap to AOC’s defense while downplaying the atrocities she enables. They preach 'principled' human rights rhetoric, only to treat genocide as just another political bargaining chip. Moral bankruptcy has never been so transparent.