Connect the iPad to OS X via a cable, launch iTunes, select the iPad in the left column navigation sidebar (if you don't have that left column visible in iTunes, then select View > Show Sidebar), navigate through the settings for the iPad to select which books or whether all books are synchronized, and press confirm.
You can also select that purchases are automatically downloaded via the iOS settings, if you would prefer. This settings is buried under Settings > iTunes & App Store > select the behavior you want.
There's also an intro to your iPad available from Apple, if you've not already seen that.
Alternatively, see various entries un the "more like this" section off in the right column of this web site.