I recently passed my Amateur Extra license exam. I had a couple of books, which were great, but I had also been using HamStudy (https://hamstudy.org/). It has proven to be a great resource (I especially like the Flash Cards). If you are thinking of upgrading your license, or getting one for the first time, I would highly recommend HamStudy.
Note: For the current Generals that may not be able to upgrade before the Extra question pool changes on July 1st, HamStudy has recently added the new Extra questions in addition to the current ones. Just be sure to choose the right one for you.