Adding third-party books to iBooks is painful. You need to cable your device (iPhone, iPod Touch or iPad) with a computer to sync books between iTunes “Books” folder and the application. I personally feel stressed every time I connect a device to iTunes. Syncing is slow and when it’s finished app icons are messed up almost every time.
Comparing to Kindle’s 60 seconds, a cable connection is just outdated. The other thing is that with the iBookstore offering such a little selection of books, many people try to download their own book libraries collected before.
Two tips listed below apply only to pdf documents, so probably only iPad owners will want to try it. You can’t use them to send ePub files to iBooks. I hope it will change soon. So far only Stanza is capable of opening external ePubs (as well as pdf docs as you’ll see in the screenshots).
I assume a book is already on your computer. What you can do is to:
1. Attach to email
When you send yourself an email with the attached book and open a message in a native Apple Mail application on your iPad, you are able to open and save it to iBooks.
2. Open from Dropbox app
If you have a Dropbox account and the application is installed on your device, you can open it and look for the pdf previously uploaded from a computer. In a latest update Dropbox added a possibility to open documents in third-party applications.
Writer Updated 
This method works for any e-reading application. EPubs go to Stanza, Kobo, etc. Mobi files can be opened by Kindle. One thing: no DRM.
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Hi, Could you please help me. I am trying to get an ebook that I have downloaded on bluefire to open in the kindle application on my ipad. Any suggestions would be appreciated.
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if you bought a book and it’s with Adobe DRM, you won’t be able to open it
in Kindle app. If the book has no DRM, you’ll need to convert the file to
mobi format. You can use Calibre for that: http://calibre-ebook.com/ and add
it to Kindle app using one of the ways described above.
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