Books on Soviet/Chinese history?

I consider myself a Marxist but I have really only engaged with literature focused on critiquing capitalism, not suggesting alternatives, but I am admittedly far too ignorant on the histories of the two largest ostensibly communist projects ever. I am not necessarily looking for books that are explicitly Marxist (although I welcome those recommendations too), I just want to get a more concrete sense for what went on in those countries during their turbulent decades.