How do I remove a Wi-Fi sync with a laptop that I no longer have in my possession?
If you're only worried about losing items purchased through iTunes on the Apple ID you're currently using, like iBooks, then this will work:


Make sure you're logged into iTunes with your Apple ID. (The one you made all of your purchases with.)
Sync the iPad to your new computer. Yes, the existing content will be deleted.
From within iTunes, navigate to the iTunes Store. In a column to the right, you will see a link (under the heading, "Quick Links" that says, "Purchased." Click on that link.
The new store page that will be displayed is broken down into sections, which you'll see across the top (Music, Movies, TV Shows, Apps, Books). You should be able to re-download any content you previously purchased, including books. I just tested this by deleting a book I previously purchased, and I was able to re-download it this way.