The most basic way to sideload is to hook your ereader up to your computer via the USB cable. Most ereaders (but not iOS ones) will appear on your desktop just like any other external drive.
Just drag your ebook file (in the appropriate format, of course) to the Documents folder on the ereader (actually NOOK is not very picky about where you place it), then eject your disk, and you can find the new ebook installed. (Nook and Kindle Fire put your sideloaded books in the My Files/Docs area, respectively.)
On iOS (iPhone/iPad/iPod touch), the process is a fair bit more laborious. You're supposed to use iTunes, as I describe in EPUB Straight to the Point, either syncing manually or automatically. But it takes longer than it should. Apple offers a little app called Book Proofer to folks through its iTunes Connect program that lets you edit a book on your computer as you simultaneously preview it on your iPad or iPhone, but it's not public, and it's not foolproof.
It turns out DropBox is a brilliant alternative to all of this. Dropbox lets you create a folder on your computer that is automatically copied to the "cloud". That means that you can access it from any other computer or even reading device.
(If you don't already have a Dropbox account, you can follow this link to get one and you and I will both get 500Mb additional free disk space.)
To sideload a ebook with Dropbox, first copy the ebook to your Dropbox folder. (If you don't know where the folder is, you can use the Dropbox menu at the top of your screen to open it: , or just find it yourself, generally right in your home folder.
Then, on your iOS device, download the Dropbox app, launch it, and sign in.
Select the ebook that you want to sideload from the Dropbox menu. It will start to load.
Dropbox can view some file formats right in the app itself, but not EPUB or mobi. So when it gets done copying the file, it will tell you it doesn't know what to do with it. Thankfully, all you have to do is click the "Away" icon in the upper right corner of the screen, and Dropbox will offer to transfer the file to an app that can view it, like iBooks in this case. Bravo, just what we wanted.
Once you choose iBooks in the menu, iBooks will open automatically and show you your book. It's really lovely:
Dropbox will work on iOS for both EPUB and Kindle/mobi:
This is pretty interesting, considering that before Dropbox
I thought Dropbox might even be the answer to the conundrum that you can create a Kindle book with audio and video, but there is no way to test it, because there is no way to get the AV book to Kindle app for iOS which (ironically) is the only Kindle app that supports audio and video! — you can only see it *after* it is published to Amazon, which, sadly, still does not allow independent authors and publishers to sell books with audio and video.
Alas, I have not been able to get this to work. I can get the audio/video Kindle books to the Kindle app on iOS, but the audio and video do not play. I haven't finished poking at it yet, though. I'll keep you posted.
Note that some ebookstores make changes to books as they deliver them. That is, a sideloaded book is not always identical to its purchased cousin, though in my mind, it sure should be. In particular, I've heard complaints about NOOK and Kobo, and I'm wondering if Amazon doesn't bake something special into its AV books for iOS as well. More on that another day.