The Mythical Man-Month by Frederick Brooks Jr. is the classic software engineering text for building complex software systems. The book was orginally published in 1975 and is based heavily on lessons-learned from the developments of IBM's OS/360. But despite its age, many of the lessons from The Mythical Man-Month hold true today.
There are two concepts I'll touch on that really resonated with me. The title mythical man-month, or perhaps in more modern terminology the FTE, or Full-Time Equivalent (or other similar term). This address the idea that people and time are not interchangeable. That adding more people doesn't necessarily speed up a project if those people don't have sufficient understaning or the tasks cannot be paralellized.
The second lesson I'll share is: plan to throw out the first one. This is as true today as it was in 1975 and is arguably more true in the context of modern agile software. We rarely get a product right on the first pass and it's through incremental iteration that we make it the better.