Both Manhattan SC and PowerScore SC are about the same - you should not get both as you won't get much out. If your SC's are still low, you should either review the Manhattan Book again or take a step down and drill into Kaplan Verbal Foundations - that book is SC only. If you are an international student - then it is a must.
For RC's - you have Kaplan and if you want to, you can get Manhattan RC book, but it has pretty much the same strategies as Kaplan except more detailed - about as detailed as Kaplan's Verbal Workbook.
Use tests wisely - use them as a measurement of your progress. If you are still not hitting the right marks - you have to go back to grammar and strategies. If it is CR's - start practicing on each area. For RC's - you may have to pick up a reading habit to improve your skills - there are strategies but no tricks for RC's unfortunately.
You can get LSAT book but if you are not hitting GMAT marks, then it will only frustrate you and do no good. If you are not winning at an easy video game - why try a harder one thinking you would do better?