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