I've just returned Principles of Modern Thermodynamics to the library. However, it is a terrific book that allowed me to understand thermodynamics that I have never would had I not run across this book. I have tried to purchase it, but it seems that this book is out of print. Therefore, my question is: are there any vendors that might offer officially out-of-print books for sale?
There is a number of websites that specialize on out-of-print or rare books. Those sites include BookFinder, used books section on AddALL, rare or out-of-print section on The Bookloft and others. In your particular case, the book that you're looking for (the 1995 edition of B. N. Roy's Principles of Modern Thermodynamics) can be easily found for sale on Amazon UK site via BookFinder and via AddALL. Here's the resulting URL: http://www.amazon.co.uk/dp/0750300191.