There are 12 different titles available at the moment from a couple different series: The Major Cities of the World and The Wonders of Nature. The ebooks are reportedly available via the Happy Studio app for Android, iPad and iPhone. That app doesn't appear to be available in the US (I could not install it) so I cannot confirm this.
This free ebook program follows a similar program in the UK. Back in January 2013 The Bookseller reported:
McDonald's has launched a two-year children’s books campaign, committing to "hand out more than 15 million books by the end of 2014” through a £1 book offer on its Happy Meal boxes.
The "Happy Readers" offer, which has the backing of the National Literacy Trust, enables customers to buy books for £1, with a token redeemable on selected books at W H Smith. The offer will be featured on Happy Meal boxes until the end of 2014.
The UK program is still going on.