Once you're public domain, you stay there. That's part of the reason why there's such a current mish-mash, with some songs written later being royalty free, and earlier tunes still under copyright.
For Robert Johnson, his music was from the early 30s, so the original copyrights would have lasted until the late 1950s. If the copyrights were renewed (the "if" is only because I'm not researching whether or not they were), that would take them to the late 80s... and more importantly, would have put them under copyright when the law was changed in the 1970s - that change was to author's life plus 50 years for the term. Since Johnson died in 1937, his works should now be public domain - I say "should", because there's a chance that some were copyrighted under a corporate ownership (like a publishing company), and that term is now 75 years from the date of creation - so any that are still under copyright should be public domain within the next year or two.
For the fakebooks, here's where it gets interesting - what they're copyrighting is their representation of music that's in public domain. I've written in other threads about the extreme number of typos I'm finding in fake books recently, and I've come to the conclusion it's deliberate - it gives them something to defend. So while O Susanna! is public domain, they could claim copyright to something very similar - and if you re-printed their version, the lawyers will come a calling.
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