New works enter the public domain every January 1st. This means that works that have been under copyright for 95 years now become part of the public domain and anyone can use these works for free. The law was changed in 2001 from 75 years to 95 years. 2021 was the first time new works have entered the public domain in 20 years.
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Search for children's books published before 1926 in the Purdue Libraries.
Most books available online (for free) are older titles - published before 1923 and in public domain.