The easiest way is to get the free Dropbox app (plus, you get up to 2GB free storage). You have to download a Dropbox program to your PC, but once it's there, you can drag-n-drop an e-book into the Dropbox file folder (it's like a regular file/folder in your PC - defaulted to be under My Documents).
Once it's in your Dropbox folder, you open the app and, depending on what type of ebook it is, you select the app you want to use to open the item.
PDFs will open in the Dropbox app's view window or you can choose to open them in Goodreader or iBooks. Epubs have to be opened with either Stanza, Nook or Bluefire Reader. I've not named a bunch of ereader apps you can use, mainly because there are a LOT - all free at the App Store. The main thing is that Dropbox will not open an epub file, you'll have to use another app for those files. Oh, and I don't know about mobi files, 'cause I don't have any of those...
For a more complicated answer, if you have a large e-book collection, and you use Calibre to manage it, there is a way (using Calibre2OPDS) to put your library into Dropbox and create a catalog that you can use to access all your e-books.
Hope this helped.
Marilyn