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The Namesake would cover this. (However, it's such a ubiquitous titling convention that the page limits examples to not-immediately-obvious namesakes.)
"If it ain't broke, don't fix it. And even if it is broke, just ignore it and maybe it'll be sort of OK — like the environment."Yes, it is a form of The Namesake, but it's a sufficiently specific form, with probably enough examples of its own, that it seems it should have a category of its own.
Well, Fictional Document is the book-within-a-book trope. I can't seem to find what you're looking for in Title Tropes but it could probably be its own trope, yeah.
"If it ain't broke, don't fix it. And even if it is broke, just ignore it and maybe it'll be sort of OK — like the environment."It's not listed on the 'Neverending Story' page either. I think YKTTW is the way to go.
Complicated - because simple is simply too simple.

Is there a trope for a title taken from a book that gets its title from a book that exists within its story? The King In Yellow is the oldest example I can think of. The Manual Of Detection, The Way Of Kings and The Hitchhikers Guide To The Galaxy are a few more recent ones.
This doesn't have to apply to books specifically, but I don't know of other examples right now.
Edited by ZaklogtheGreat