Alexander Koryagin asked:

AK> President Trump unveiled his 2019 budget plan, here are
AK> five takeaways that will affect the American people.
AK>
AK> Trump's 2019 budget book is called "An American budget".
AK>
AK> Why "an" -- not "the"? After all, we know it is about
AK> the US budget for 2019. I'd say "The American 2019
AK> budget" or "The US' 2019 budget".

I think Trump implies that the budget is American in spirit.
The budget he proposes in an American one.  In this sense,
"the" would be inappropriate because, obviously, it is not
the only possible American budget.

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