Your profile doesn't say where you are, but http://support.apple.com/kb/HT4059 says :
The availability of the complimentary book varies from country to country or region. If you misplaced your complimentary copy of "Winnie-the-Pooh," you may be able to redownload it for free from the iBookstore if the book is currently available in your country or region
If it's not on your bookshelf in iBooks, and you've tried searching for it via the Store button in the top left corner of iBooks and can't find it, then it may not be available in your country.