Genevieve Cook says she met the president at a Christmas party in 1983 when he was 22 years old.
In an excerpt obtained by Vanity Fair Cook says she found Obama charming and alluring yet distant and unreachable.
In one entry, Cook writes, "The sexual warmth is definitely there. But the rest of it has sharp edges and I'm finding it all unsettling and finding myself wanting to withdraw from it all. I have to admit that I am feeling anger at him for some reason, multi-stranded reasons. His warmth can be deceptive."
Cook's diary entries will be in a new biography by David Maraniss called Barack Obama: The Story. It will be published next month.