Get an iBook from the hard copy
For things you've bought already, not easily. You can look online for PDF versions of the book which aren't strictly legal, but apart from that you don't have a huge number of options. 

However, I've definitely seen books that come with their digital copies - although the last one I bought, which was a Wrox book, required me to install Adobe Digital Editions.

Another option, if you have developer books, might be Safari Books Online, which allows you to save up tokens to download full copies of books.