The difference is probably based on publication date and cover type. Every new printing gets a new ISBN, and Hardback and Paperback editions are of course different.
Here is a link to 0304353302:
http://www.abebooks.com/servlet/Sear...304353302&x=25
And here is 030681160X:
http://www.abebooks.com/servlet/Sear...30681160X&x=59
And a general section with all available copies:
http://www.abebooks.com/servlet/Sear...n+Coffins&x=56