Many people write to ask if this or that is public domain, meaning not subject to copyright enforcement as traditionally understood.
There are two essential links: The copyright flow chart and the Stanford Renewal Database.
Believe me, it was not always this easy!
This is for text and music but remember that most music collections involve more than one contributor. So there are complications to the process. Rights typically reverted to the author back when things went out of print (nothing goes out of print in that sense anymore). There are also international issues that can introduce provisos. In general, however, I’ve found that far more books and music, and music books, from pre-1964 are available that is usually supposed.
Another thing to remember is that a fight over these matters is virtually never worth the trouble, so if so silly publisher claims copyright over engraving – not words or music but just the typesetting – one usually has to relent to the claims of the deep pockets and then move on. Still, it is not the case that you just have to take a publisher’s word for it on all matters. Do your own research and act within the law and all is well.
Also, it turns out that you have to search anywhere within four years of the renewal date (which came up during the Greg Bear/Poul Anderson thing just recently).