You can configure iTunes to automatically download the purchases (apps, books, music) made from the App Store, iTunes Store....
The procedure is as follows:
- Open iTunes and sign-in using the Apple ID (used for the purchases).
- Click on the Store menu > Authorize This Computer (Apple ID required).
- Click on the Edit menu > Preferences > Store.
- In the Automatic Download section, check the following items:
- Click on Ok to validate.
The procedure for iOS is explained here: http://ccm.net/faq/34438-ios-automatically-download-purchases-made-on-other-devices