I've just finished Ernest Mandel's Marxist Economic Theory. It was a long and extensive read (800 pages) but completely worth since it personally made socialist governance on the technical side easier to comprehend. The author prefaces right from the start with the fact that this book is not a compilation of quotes from Marx and co and an exegesis thereof, instead it goes over economic laws and mechanisms based upon contemporary, academic works and quantitative data. The only caveat is that the book was published in the 60s and so it is quite outdated (especially when addressing the Soviet economy).

Ernest Mandel is a Belgian economist who seems Marxist-Leninist enough to support and understand the USSR from a political viewpoint while providing (what I found to be valid) criticism on certain economic policies during Stalin's and Khrushchev's office.