By definition, there is only one easiest way to do anything.
Long standing open publishing leader lulu.com appears to have the easiest way of getting content into the iBookstore.
Apple
For the Apple way, start here..
-- https://itunesconnect.apple.com/WebObjects/iTunesConnect.woa/wo/0.0.0.5.7.7.1
Lulu
For lulu, start here..
-- http://www.lulu.com/apple-ipad-publishing/
Lulu is the brainchild of Red Hat Software co-founder Bob Young. Bob Young's accomplishments and dreams are outlined in a recent blog post on localtechwire.com, where local refers to the Research Triangle Park area of North Carolina.
-- http://localtechwire.com/business/local_tech_wire/opinion/blogpost/7519260/
Enterprise Content
Getting enterprise content onto the iPad, without going through anybody's store or application procedures, is easier still. The easist way IMHO is using Calibre as the <your format> to epub intermediary. Details here..
-- http://ipliance.com/index.php/eng/Blog/Roll-Your-Own-iBooks-for-the-iPad
iBooks in List View on an iPad