computer to ipad transferrs

It depends on what format of a eBook you downloaded. The iPad, specifically iBooks, supports epub and pdf formats. If you open up iTunes you can do a "File --> Add to Library" and choose those files. That will then make them available to sync to your iPad.

The other thing is that you can use Good Reader, an iPad app, to download things directly from the web. For example, if I buy something from O'Reilly I can use the Good Reader web browser to navigate to the download page and retrieve the book/document. There is then an option in Good Reader to "Open in..." the file into iBooks.

CR

On the iPad you can log in on the Oreilly website and download the epub version. the site will ask you if you want to open in iBooks.. done!
or from the iMac you open iTunes and you transfer it either via USB or wirelessly if you set it up that way.