ibooks

What sort of ebooks are they ? If they are ibooks or DRM-free epubs then you should be able to add them to your computer's iTunes library via File > Add To Library (if using a Mac with OS X Mavericks on it them File > Add To Library in the iBooks app on it), and you can then sync them to your iPad via the its Books tab

If they are not DRM-free epubs then does the store that you bought/downloaded them from have their own app in your country's App Store which you can download and read them in ?