Read epub book purchased from iTunes on Windows
Depends on whether it's protected by DRM or not. If it's not DRM protected, you can use free software like Calibre. You can also read the ePub books in Firefox using the EPUBReader extension and in Chrome using the MagicScroll eBook Reader extension.

To know if your eBook is DRM protected (fixed the description to correctly say "DRM" instead of "copyright"):


  
  Open iTunes.
  Select the target ebook.
  Choose File.
  Choose Get Info.
  Look at the “Media Kind” field.
  
  If it says “book,” it is a web download.
  If it says “purchased book,” it’s a DRM free iBook purchase/download.
  If it says “protected book,” it has a DRM attached.