The Mythical Man-Month: Essays on Software Engineering
The fundamental idea is that there are two types of time: (1) Calendar time; i.e. the time that can't be sped up with more people, e.g. no matter how many women you assign you can't grow a baby in less than nine months. (2) Time; how people generally think of time, if something takes a year for one person, surely you can do it in a month with 12. E.g. if you're doing the dishes, it might go faster with two more people, one to dry and one to put things away (and those people are likely idle 50% or more of the time), but at some point, it's just going to be slower, and the number of kitchen sinks limits you.
This idea is essential, and Brooks certainly is responsible for introducing this idea into the software realm. However, the contents of the book haven't aged well. Likely not worth your time anymore.