RubyCocoa is to interface with the Cocoa API.
Cocoa is BIG.
The ability to open and read contents of .doc files is there.
Your mileage will vary of course, depending on what features of what  
versions of Word are included in the file.
The Cocoa AppKit class NSTextView can display the contents easily.
You've got an uphill battle though, lots to learn to do it. It all  
depends on what you want to do with it though.
You could easily use rb-appscript with the application TextEdit  
(bundled on every OS X system with open source bundled with XCode).
That should get you enough to see if it is worth it to you.
.doc is one of many file formats native to OS X, but again, your  
mileage will vary.