If you have never synced your iPad before, iTunes will not recognize your iPad and will want to erase everything that is on it and replace it with whatever is in iTunes on your computer.
You need to make your computer your syncing computer first. This helpful user tip explains very clearly how to go about accomplishing this.
You cannot sync photos on your iPad into iTunes. You have to import them onto the computer using whatever camera application that you use - it is just like importing photos from a digital camera. This will explain that in more detail.
Have you synced the iPad to that computer before ? If not then you might this articla useful : https://discussions.apple.com/docs/DOC-3141
You can copy your iTunes purchases, including ibooks, via File > Transfer Purchases (File > Devices > Transfer Purchases if you have iTunes 11 on your computer).
To copy photos to your computer that were copied to the iPad via the camera connection kit or saved from emails/websites then see this page - on a PC you can use the windows camera wizard, on a Mac you can use Image Capture, iPhoto or Aperture. To copy photos that were originally synced from a computer you will need a third-party app such as Simple Transfer which can copy them off via your wifi network. But as photos are 'optimised' when they are synced to the iPad, any that you then copy back to a computer may not be exactly the same as they originally were on your computer.
If you are updating the iOS version then before you start the update you may also want to copy any important documents off the iPad as well e.g. via the file sharing section at the bottom of the device's apps tab when connected to iTunes, via wifi, email, dropbox etc - they should be included in the backup, but it's best to have a copy of them outside of the backup just in case