If you are looking for prep materials for reading comprehension, I recommend using the Manhattan book. Manhattan publishes the best resources in their books, and you are guaranteed to have high quality questions. And of course, in terms of practice, you can't do better than the Official Guide to the GRE. Since this book is made by the test makers, it has questions that are more similar to the actual test than any other test prep book. Besides that, there is not too much else out there in book format that will be useful. One other tip, though, if you are looking for extra practice or challenging practice, I recommend using the GMAT Official Guide. The GRE and GMAT Reading Comprehension questions are extremely similar, so you would definitely benefit from practicing with some of the GMAT material.
I hope that this helps a little.