I'm sure many here have encountered a common error in written English, whereby 'have' is substituted by 'of'; 'should of', 'would of', 'could of', etc. It's my understanding that this is always grammatically incorrect, and derives from when people contract 'have' to 've' ('should've', etc.) which sounds a bit like 'of'. However, there are a couple of instances of this error appearing in books I've read, most recently The Great Gatsby.
On page 32 of my edition (Penguin Classics, 2010), Myrtle says:
"I had a woman up here last week to look at my feet, and when she gave me the bill you'd of thought she had my appendicitus out."
Likewise, on page 74, Wolfshiem says:
"It was four o'clock in the morning then, and if we'd of raised the blinds we'd of seen daylight."
To my mind, these instances should use 'have' instead of 'of'; "you'd have thought...", "if we'd have raised...we'd have seen...". So either;
- I'm wrong about this being bad grammar.
- It's a grammatical use which was acceptable when Fitzgerald was writing but no longer (though that doesn't explain the same error in Terry Pratchett's and Neil Gaiman's Good Omens).
- It's an error with my copy.
- It's a deliberate error to indicate a 'lower' use of language by these specific characters to contrast with the likes of Nick, Gatsby, etc, like how Myrtle says 'appendicitus' when she means either 'appendicitis' or 'appendix', or Wolfshiem calling Oxford 'Oggsford'. (This might also explain the use in Good Omens.) If this is the case, it's quite subtle for what is supposed to be dialogue.