::::::::::::::::WP:SCHOLARSHIP is the relevant guideline. Any scholarly work could be reliable. We can't use them all, but can be guided by which ones are cited most often, have remained in print longest, from most notable authors. And ensuring that we make a fair sample of the various kinds of relevant work, Marxist, non-Marxist and anti-Marxist, probably with more Marxist than the others because that's the largest body of work.