I think it depends on your goals. If you want to improve your rhythm playing by yourself then definitely Strumming and Rhythm Mastery. If you want to improve on being a rhythm guitarist in a blues band then I would recommend going through the rhythm chapters of Blues Guitar Unleashed and/or 5 Easy Blues Songs. If you want to improve on being a rhythm guitar player in a rock/classic rock band, then I would check out the rhythm chapters of Classic Rock Unleashed.