The American co-author of a book titled Fiesta: How to Survive the Bulls of Pamplona has been badly gored in the third bull run of Pamplona's San Fermín festival.

Bill Hillmann, 32, from Chicago, was gored in the right thigh, the Navarran regional government said in a statement. Despite facing off against the heaviest of Wednesday's six bulls – weighing about 600kg (1,300lb) – his injuries were described as serious but not life-threatening.

The injury comes just one month after Hillmann's book was published. Described as "the ultimate guide to surviving" the festival, it offers tips on the morning bull runs, in which runners dressed in white outfits and red scarves dash through Pamplona's narrow streets pursued by fighting bulls, as well as on how to enjoy the nine days of partying that draws tens of thousands of Spaniards and foreigners to the Navarran capital each year.

Another of the book's five co-authors, the British writer Alexander Fiske-Harrison, said that Hillmann underwent surgery the same day. Writing on his blog, he said that he had visited Hillmann in the hospital and his friend seemed to be doing well. In a previous post he highlighted Hillmann's decade of experience in bull-running, calling him the "best young American runner on the streets today".

Another co-author, John Hemingway, is the grandson of Ernest Hemingway, who shone a spotlight on the San Fermín festival in his 1926 novel The Sun Also Rises.

A 35-year-old Spaniard from València, identified only by the initials of JRP, was also listed as being in serious condition on Wednesday after being gored in the chest, the Navarran government said. After a hair-raising run that saw one bull separate from the pack and charge at several runners, three other Spaniards were also admitted to hospital with bruises.

Fifteen people have been killed in the festival since records began in the early 1900s. Dozens more are injured each year, many of them during the panicked, chaotic stampede of the bull runs.