The Bath Massacre: America’s First School Bombing by Arnie Bernstein is an excellent book about the disaster which covers everything from the town itself to Andrew Kehoe’s life from its start to its end.
The other book written on the subject is The Bath School Disaster by Monty Ellsworth, a neighbor of Kehoe who knew him for many years. The book was written shortly after the disaster and while Ellsworth is no professional writer, he did a good job of gathering what information he could to produce the first book available to those curious about what happened in Bath.
A transcript of the coroner’s inquest into the death of Superintendent Emory Huyck is provided in full at the link. The inquest occurred the day after the disaster and has eyewitness accounts given quickly enough that the memories were still fresh in everyone’s minds.