I had the same problem. I took the Veritas course and their Reading Comp book is hands down the best for RC that I have ever read. The problem is they don't sell the books individually. I would recommend trying to find someone that has taken the course and see if s/he will sell you the book.

Doing the problems is great, but the learning happens in analyzing the problems and discovering mistakes and patterns. Are you sure you need another book? Perhaps it might be helpful to categorize your RC errors? What kinds of questions are you missing? What types of passages challenge you? There are lots of great articles out there by Stacey with MHGMAT that tell you how to analyze your work. This has really helped me. Maybe if you can recognize your RC errors, than you can address them with resources and questions in the forum? Also, I worked with a tutor who uses the LSAT Superprep guide. There is an explanation in the beginning which is really good and talks about approach. I hope this helps some and I wish you continued success in your gmat efforts!