Marketing blurb aside, the main point of that post is:

Here’s how it works.

1) Someday soon they will announce the release of Unity iOS Game Development Beginner’s Guide from the @packtexplorer twitter account using the code #unityiosRT

2) The first 100 unique re-tweets get the book free!

3) You need to be a follower of @packtexplorer (if you’re doing gamedev you probably should be following @packtexplorer anyway!) so that we can contact you.

4) You need to have a Packtpub account with your email and address so that we can credit your book to that account.

5) A couple of days after the announcement we will list the twitter @username of the 100 winners. If you’ve won, send a DM (twitter direct mail) to claim, including the email you used to set up your Packtpub account.

6) The book will appear on your account for you to download as a pdf or epub on your tablet or ereader.

So watch @packtexplorer for the announcement with the code word #unityiosRT