http://www.copyright.gov/circs/circ1.html#hlc
Quote:Works Originally Created and Published or Registered before January 1, 1978
Under the law in effect before 1978, copyright was secured either on the date a work was published with a copyright notice or on the date of registration if the work was registered in unpublished form. In either case, the copyright endured for a first term of 28 years from the date it was secured. During the last (28th) year of the first term, the copyright was eligible for renewal. The Copyright Act of 1976 extended the renewal term from 28 to 47 years for copyrights that were subsisting on January 1, 1978, or for pre-1978 copyrights restored under the Uruguay Round Agreements Act (URAA), making these works eligible for a total term of protection of 75 years. Public Law 105-298, enacted on October 27, 1998, further extended the renewal term of copyrights still subsisting on that date by an additional 20 years, providing for a renewal term of 67 years and a total term of protection of 95 years.
Basically 75 years for anything that expired before 1998 but after 1976, 95 years for anything copyrighted before 1998 but hadn't expired then.
70 years after the death of the author for anything newer than that.
But in the case of Wizard of Oz, first published in 1900, it is no longer under copyright.
This from Wikipedia.
Quote:The book has been in public domain since 1956.