The former German chancellor described Trump as "emotional," and questioned his fascination with Russian President Vladimir Putin and other authoritarian leaders, Biden writes in the memoir.

Biden writes that Merkel was particularly frustrated by Trump's behavior during their meetings, including his refusal to shake her hand, and his apparent preference for personal impulses over established facts.

"In our conversations ... Angela was candid about her frustrations with my predecessor, who refused to shake her hand, and who seemed 'emotional' in their meetings, driven by mood swings rather than facts," Biden writes.

"She also wondered why he was fascinated' by Vladimir Putin and other politicians with autocratic and dictatorial traits.' That was a mystery I could not unravel."