If you are looking for an iPad learning program, with the fun experience and benefits associated with built-in dictionary, word translation and search terms, the Amazon Kindle application for the iPad is the best choice.
Apple has its own selection of iBooks, in addition to having books and documents of any kind synced and associated with your iCloud account, which ensures you can access it from any program.
However, the tasting and learning experience provided by Kindle is non-competitive, especially if you also have an Amazon Reader, or subscribe to the Amazon Unlimited service.
You can download Kindle for iPad for free by clicking on this link.
