In the meantime, here are some tips to help you find that elusive book:
Visit AbeBooks
If you're looking for a rare or out of print book your first and best option is AbeBooks.com. They also have a BookSleuth facility for help in identifing a book.
Use Libraries
such as The British Library or WorldCat to find out useful information to help your search.
Place a want
at Sheppard's confidential. Their newsletter is read by many book dealers.
Use a search agency
such as Hoovey's Books who seem to exist solely to source books.
Find it at AbeBooks.com, the world's premier site for used and rare books...