Bill Hillmann tripped during the bull run at the Pamplona festival on Wednesday and was set upon by one of the charging beasts. He reportedly "suffered two horns."
In addition to Hillmann, 32, a 35-year-old man from Valencia was also badly gored by one of the bulls, according to the festival's website.
Hillmann, who co-authored the book with John Hemingway, Joe Distler and Alexander Fiske-Harrison, is an practiced runner with 10 years of experience under his belt, sanfermin.com reported.
Fiske-Harrison wrote on his blog that "Buffalo" Bill Hillmann was doing well after surgery.