I may have mentioned these before, but they have just come up again in conversation for me as examples of nonfiction books that explain technical subjects very, very well.
I am rereading "Why Does E=MC^2" by Brian Cox and Jeff Forshaw. It is one of the few popular science books that actually explains fundamental things in a way that makes sense to me. For example, it explains why the speed of light is exactly what it is and not slightly faster or slower. A lot of science books are lists of facts with some philosophical speculation thrown in and the occasional "This will blow your mind" chapter. This book is not like that.
Similarly, "What Language Is" by John Mcwhorter actually explains real linguistic concepts and walks you through precisely how it is that, for example, grammar and vocabulary shift in a language over time. He doesn't just tell you that these things happen but he walks you through specific examples of, for instance, a language acquiring new inflections or shifting some fundamental aspect of its grammar.
Finally, "Why Does the World Exist?" by Jim Holt is an utterly fantastic account of different theories of the metaphysics of existence. Everyone should read it.