This is a great book about the empirical research on unions. Though it is somewhat dated. One of the key things to understand from this book is that unions, aside from benefiting workers with higher safety standards, more time off, and higher wages, they also give workers a *voice*. To have a voice means to have a say in how a workplace operates, to have your concerns heard, etc.
One of the main conclusions I drew from this book is that, in broad strokes, unions are an overall force for good in society from nearly every angle. Their only problem is that they reduce a businesses profitability (but not its productivity!) and are thusly demonized by corporate press and in the popular imagination.
A good paper to read along with this book is: Brimeyer, Ted, Eric O. Silva, and R. Jolene Byrne. "What do unions do? Media framing of the two-faces of labor." WorkingUSA 19.4 (2016): 517-532.