Sometimes works under copyright protection (works published after 1930 or post 1930 editions and revisions of pre-1930 works) go out-of-print. This means they are not easily obtainable from the publisher or major distributors. Sometimes these works can be accessed through inter-library loan or purchased as used copies from people who have them or companies that deal with used books or other materials.
Remember that being out-of-print does not mean losing copyright protection. Out-of-Print materials are still under copyright protection. They may or may not be published again. If you're using an out-of-print material, you still have to follow Fair Use guidelines or ask for permission from the copyright holder.
Out-of-print / obsolete media materials
Copyright law (section 108) addresses out-of-print and obsolete formats for media materials. In general, you cannot change the format of a media material without permission from the publisher. If the publisher is no longer active, the law allows libraries to make up to three copies of an out-of-print and obsolete media material for preservation only. The material cannot be used outside of the library building. There is no streaming audio or video format option. The library can make preservation copies on compact disc (CD) for audio and DVD for video from obsolete and out-of-print materials. It cannot digitize materials and upload them online in streaming format.
Please note that as of the date of this writing (2019), audio cassettes, long playing records (LP) and VHS tapes have not been officially declared as obsolete formats since their playback equipment are still available in the market. UIW no longer supports playback equipment for these formats.
The following link is a spinner of preservation guidelines for out-of-print materials.