Democratic presidential front-runner Hillary Clinton is facing "mounting health issues" and worries she won't be able to continue in her quest for the White House, author Ed Klein writes in his new book "Unlikeable — The Problem with Hillary." Radar Online published excerpts from the book on Tuesday that say the 67-year-old former secretary of state is "frequently plagued" by "blinding headaches."Clinton also has suffered a series of strokes during her campaign, Klein writes."For the first time I’ve known her, she’s showing self-doubt about her strength and vitality," Klein quotes a Clinton friend. She also is suffered insomnia, which has sapped her energy, according to Klein.Clinton tried taking sleeping pills, but, "She said they made her less sharp the next day," Klein quotes the friend.She also reportedly fears another blood clot, like the one she suffered in December 2012."There were incidents on the campaign trail when she felt faint and nearly swooned," Klein writes. "Those incidents were kept secret."