Short answers, pulled from the story.
The first three chapters draw on the real 10th-century travel account of Ahmad ibn Fadlan, a diplomat sent by the Caliph of Baghdad. The rest of the novel retells Beowulf, with ibn Fadlan as an outsider observer. Crichton acknowledged both sources in an appendix.
A friend was building a college course called "The Great Bores" and included Beowulf in it. Crichton argued Beowulf was genuinely exciting. The dispute escalated until Crichton set out to prove his point by publishing a retelling in 1976.
The novel was republished under that title to match the 1999 film adaptation. In the story, ibn Fadlan is recruited as the 13th member of the Viking company because a soothsayer required exactly that number for the quest to succeed.
Yes. Ibn Fadlan was a real diplomat who traveled north in AD 922 on a mission from the Caliph Al-Muqtadir. His personal account of that journey, including observations of the Varangians, forms the basis for the novel's opening chapters.
The 13th Warrior earned about $62 million worldwide against a production budget that exceeded $100 million. The financial shortfall placed it among the notable box-office losses of its era. Critical response was mixed.
John McTiernan directed the film, but Disney fired him partly because the production went far over budget. Crichton then directed a reshoot without taking an on-screen credit. Despite the troubled production, Crichton wrote that he was quite pleased with the result.