If I'm going to read Power and Market, do I need to read Ch.12 of Man,Economy,and State?
It's a separate entity and does not require a reading of chapter 12. This is in large part because Power and Market was just supposed to be the last chapters of MES.
With this said I implore you to read MES before Power and Market. With an understanding of how Rothbard frames the free market you will have an infinitely easier time understanding P&M
I'm reading Man,Economy,and State right now. I just asked because I remember reading in Justin Raimondo's biography of Rothbard, An Enemy of the State:The Life of Murray N. Rothbard, that the publisher for Man,Economy,and State told Rothbard to cut out the final part (and apparently it was compressed into one chapter), which was later published as Power & Market.