Is it possible to read iBooks without an iOS device? I no longer have an iPhone or iPad, but still have books in my iTunes account that I would like to read. Is there a program for OS X that will allow me to read iBooks without an iOS device?

I am familiar with calibre, but the main issue here is the DRM.
OS X 10.9 Mavericks includes an iBooks app that allows you to read any documents - including purchased books or your own pdfs - that you had in iBooks for iOS.

Mavericks is a free upgrade available for the following systems, and was released on 22 October, 2013 through the Mac App Store.

From OS X Mavericks : System Requirements


  To install Mavericks, you need one of these Macs:
  
  iMac (Mid-2007 or later) MacBook (13-inch Aluminum, Late 2008),
  (13-inch, Early 2009 or later) MacBook Pro (13-inch, Mid-2009 or
  later), MacBook Pro (15-inch or 17-inch, Mid/Late 2007 or later)
  MacBook Air (Late 2008 or later) Mac Mini (Early 2009 or later) Mac
  Pro (Early 2008 or later) Xserve (Early 2009)  Your Mac also needs:
  
  OS X Mountain Lion, Lion, or Snow Leopard v10.6.8 already installed 2
  GB or more of memory 8 GB or more of available space