Ebooks are better than the textbooks. It is portable and can store huge learning materials. I recommend illustrate - The Video Dictionary for GRE/GMAT aspirants.
I followed "illustrate - The Video Dictionary" and recommend you to improve your vocabulary. It is available on:
iOS App Store: https://itunes.apple.com/app/id855051941
Google Play Store: https://play.google.com/store/apps/details?id=com.mocept.illustrate