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)

Mail them as attachments to your iPad. If the attachment shows up with a dotted outline, tap it to download. When solid, tap again for an Open In option to save it to iBooks.

The downside of doing it this way is that it will not be backed up on the PC in iTunes. If you have to restore the iPad you'll have to add the books again manually; so keep a copy somewhere you can find it.

If you use DropBox, you can also add the book that way. Adding it to the DropBox site on the laptop, then using the app on the iPad to Open In iBooks. If you don't mind leaving it in DropBox (storage permitting) that can also be your backup.