Do Practice Manual only, & Keep the book of Saxena Vasist next to you so If you cannot understand some concept in some topic you can understand it from S & V else Costing is a subject of analytical skills & Conceptual study, Do it with the Perception of studying Concepts in Depth & solve the whole PM of ICAI & that would be enough to get atleast 50+ in that Subject.
If you are looking for a Specific Option Btwn Ravi Kishore & JBG than RMK is Tried & tested Good Book. Even JBG is a Nice Book as but launched Just Last Year. However Dr J B Gupta is a Very highly Ranked teacher, so definately his book shoud be Good, but keep in mind its a newly launched Product