The Time Tunnel by Murray Leinster (1967)
First, let me explain what this book is not. It is not Leinster's 1964 novel Time Tunnel. Nor is it the 1956 novel Tunnel Through Time, published as by Lester del Rey but written by Paul W. Fairman from an outline by del Rey -- Donald H. Tuck's The Encyclopedia of Science Fiction and Fantasy mistakenly credited that book to Leinster.
What this book is is the first (of two) tie-in novels Leinster wrote based on the Irwin Allen 1966-7 television series The Time Tunnel. Leinster was the major pen name of Will F. Jenkins, who had a long career in both the pulps and the slicks. Although Leinster was dubbed "The Dean of Science Fiction," he was equally comfortable in a number of genres -- science fiction, mystery, western, adventure, historical, and love. Leinster seldom wrote tie-in novels. Previous to this novel, he had written only one tie-in novel, Dallas, based on the 1950 Gary Cooper/Ruth Roman western. Leinster also published the 1960 "novel" Men Into Space, which was actually a collection of short stories based on the 1959 William Lundigan television series. Leinster followed The Time Tunnel with a sequel Timeslip! (also 1967); he followed these up with a three-book series (1968-1960) based on another Irwin Allen show, Land of the Giants (1968-1970)
Full disclosure: My personal opinion of Irwin Allen as a television producer is pretty low. I am not a fan of The Time Tunnel, Land of the Giants, Voyage to the Bottom of the Sea, or Lost in Space. Allen's television credits belong in the same moldy box as Aaron Spelling's; both made a lot of money on lousy shows.
That being said, Leinster's novel is not Allen's television show! The basic premise is the same. A costly and secret scientific project in time travel is threatened with closure by an influential senator who believes the whole project is hogwash. Two men, Tony Newman and Doug Phillips, are sent back in time where they encounter various historic events and people.
The television show has the two first landing on the Titanic. Leinster ditched that somewhat time-worn scenario, instead having his heroes landing in Johnstown, Pennsylvania, just before its devastating flood. Ignoring advice from his superiors, Tony tries to warn the populace of the impending danger -- to no avail. He and Doug do manage to save an eight-year-old girl, though, and she turns out to be a direct ancestor of the senator trying to close the project.
This gives them a circular conundrum. Were the two destined to go back in time to save the girl? Are all their actions in the past preordained? If they had not gone back in time would history have changed? Those questions are still with them as their 20th century colleagues try to bring them back. Something goes wrong and they find themselves in 1874 Kansas, just outside the trading post of Adobe Wells, where the Indian chief Quanah Parker is about to lead over a thousand warriors to destroy the post and kill everyone there.
Leinster mined a lot of historical detail to make both scenarios as realistic as possible. The book reads well and is as mature as the television show is juvenile. An exciting tale that is much better than its origins and the book cover would make one think.