I usually sync eBooks (PDFs and ePubs) to my iPad1 from my desktop PC. When I'm away I want to be able to add eBooks to the iPad from my laptop, but without clearing away the existing eBooks on the iPad. I've exceeded my free iCloud quota, and don't want to pay for more.
How can I put additional eBooks on the iPad from my laptop without deleting the sync with my desktop PC (and hence losing all of the existing content)?
(Both PCs are running Windows 7)