To the best of my knowledge, the federal government has been checking on not only book titles purchased by individuals, but also what they check out of libraries ["What IRS demanded from me"].
This has been going on for at least 10 years, possibly 14. I can't remember under which administration it began.
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While I agree that government is getting out of hand, it would appear that the request is legitimate in that if you were a scientific researcher regarding DNA and your book purchases were romance novels and travel guides, those would be disallowed. However, if you provide the titles of the books you purchased, which may well be wide ranging due to the nature of your writing, if you make a case for why these are important for your research, I believe you should be on sound footing.
Do I believe our citizens should be monitored in this manner? No. But I once knew a prominent engineer who was a direct descendant of one of our presidents. He refused to file income tax returns. He was put in jail for many years, even though his reasoned and valid claim was the tax requirement is unconstitutional.
If you do stand your ground based on invasion of privacy and freedom of speech, you have my respect and prayers.