A small book, essentially based on an articles Samuel Edward Konkin III once wrote on the topic but contains a lot of new material by Wally Conger himself. It deals with non-Marxist social classes theory and analyzes. I won't paste author's conclusions here (just read the book, it's short and cheap anyway) but main idea is that Marx and Marxoids while using wrong method, wrong premises and got wrong conclusions were indeed right in some things. For example, Marx insisted that there is a exploiter class in our society, it's true, argued Konkin, but Marx was incorrect about who those exploiters are.
Well, enough, it's really short and easy so just read it even if you disagree with something. Must read for Libertarians and Left, particularly good for Libertarian Left folks, but I think just everyone should read it (maybe after New Libertarian Manifesto or/and An Agorist Primer.)