The great thing about electronic publications is that you do not need to keep your entire library on your iPad! With Amazon, for instance, all of your purchases are kept in your archive at Amazon. The only thing you actually need to have on your iPad at any given time is/are the book(s) you are reading at the moment.
When you are done with a book, you can delete it off your iPad to free up space, but should you ever want to read it again, all you have to do is download it from your archive. There is no charge for this and you can do it as many times as you like.
For other "stuff", you can also use cloud storage on a place like DropBox. As long as you have Internet access, you have access to your reading material.
Clear as mud, eh?