The stock trades at below book because nobody wants to deal with the inherit volatility of the mortgage market.
Investors want reliable, steady earnings. Mortgage companies and banks that are mortgage heavy have historically, are currently, and will ALWAYS trade at a lower multiple to the Book value.
Nobody has ever "cracked the code" for how to keep a mortgage business profitable through the business cycle. Mr. Cooper is probably doing the best possible job that has ever been done. Usually, the MSR asset helps hedge against massive losses of an origination business when the housing market cools off. It seems that Mr. Coopers heavy utilization of the Correspondent channel allows them to enjoy a scalable cost structure. Had their volume been "in house" we'd see them being crushed by restructuring costs as they try to downsize the production engine.