for a 1975 Las Vegas phone book Maybe the library in Las Vegas?

Also, here's a link to the history portion of the Las Vegas Sun website, click on the interactive casino history map...It doesn't have a page for 1975, but it does have 1970 and 1980, so might get you started:

http://www.lasvegassun.com/history/

Sounds like a fun project...good luck!