You can make ibooks accessable by posting the files on a web server and providing your users with a hyperlink to the file. Your iOS users then can use safari to download the book. When you download the file you will be prompted to what program you want to use to view the file. Just choolse ibooks and it will save the file in your ibooks app for future use.
Note: you must add the ibooks mime type to your web server so it knows what to do wtih the file when a user reqeusts it.
I have tested and confirmed with IIS.