What books can I read to learn about cultural Marxism?

Gramsci is a good intro. Dialectic of Enlightenment is needlessly difficult to read but the first essay is good for understanding the basic idea of critiquing "capitalist culture," instrumentalization e.g. Some kind of intro to Gramsci would be your best bet though.

The interesting thing is that a lot of cultural Marxist critiques dovetail with the most incisive (and much less read) right wing critiques of modernity, industrial and mechanical man, mass man, capitalist culture, etc. Not uncommon to see people smushing Gehlen together with a bunch of leftist wieners.