Calibre (free, open source software) will allow you to convert between most ebook formats, and you can convert an entire document to, e.g., rtf or plain text and open it in your Word Processor -- not to mention that its own ebook reader will allow you to copy and paste text.
That's all assuming there's no DRM.
http://calibre-ebook.com/
That's just one example. Depending on what formats these files are in, I might suggest others.