David Von Drehle's Triangle: The Fire that Changed America is an account of the 1911 fire in the Triangle Shirtwaist factory in which 146 young female workers died. To develop a thesis statement (a sentence or two asserting the major claim you need to prove), first you much choose some one specific aspect of the book about which to write.
One possible thesis would be "Although the Triangle fire occurred over one hundred years ago, the positive steps towards unionization and labor reform secured in the wake of the tragic death of 146 young women are starting to unravel in the United States as right wing ideologues attempt to roll back many of these hard-won workers' rights." Although we would like to think that especially in the wake of the horrible tragedy described in the book we have eliminated sweatshops, in fact protections for workers have been slowly eroded over the past few decades.
Another possible thesis would be "Although the Triangle fire was a tragedy for the young female victims and their families, their deaths were not entirely in vain, for the publicity surrounding them gave a strong impetus to Progressive politics and improved worker protections in the United States."