Where did the Towers go? By Wood
The author is an engineer with Ph.D. She uses math, physics and engineering principles to prove that the Twin Towers did not pancake. She says her book is not a conspiracy book. She does not discuss why or who brought the buildings down. Her point is that it was not physically possible to pancake the buildings and that it was not a controlled demolition.
I do not have an engineering background but her text is very understandable. There are plenty of photos that explain the text.
If you are interested in The Who and why, there are several books. You might start with Black 9/11 by Gaffney.
"Get there firstest with the mostest."