Open iBooks on your iOS device, then go to the store
Search for the book you'd like to link to
Tap the share button in the top right and then copy link
Open your Internet browser and go to goo.gl (optionally, sign in to your Google account to save your links for later)
After you shorten the link, on the right side of the screen, tap "details"
You will notice a QR code of your link has been created. Press and hold on the QR code.
Save the image to your Camera Roll and you're done! (If you signed in with your Google account, you'll be able to get back to this QR code later at goo.gl)