Specifically, as I've been cataloging my library, I have been tagging as well, and trying to come up with reasonable tags for the books I like about science. They are somewhat indescribable (the word wacky is the best term to date). But then I realized what connected them. They are written with a very present authorial voice. The author starts out being newly interested by a topic, but largely uninformed, and goes on a journey of discovery (investigative journalism), making the errors that an intelligent newbie would make when talking to experts in a field, and sharing the perspective of an intelligent generalist with a great sense of humor. Ed Regis. Mary Roach. Elizabeth Royte. Sometimes even "Cecil Adams". Jon Ronson (altho calling what he does science journalism is taking that term to the edge and beyond). Bernice Hausman, even Robert Karen and Fiona Giles, altho with Giles, that's moving more into the realm of another kind of book I like: the collection of true stories which represent a body of knowledge well, possibly with additional commentary (Barbara Behrman).
I also like another category of very-present-authorial-voice: the enthusiastic, evangelical expert. Dement and Rathje spring to mind. So it isn't _just_ the voyage-of-discovery. It is in large part that subjectivity in an area that is often so "objective" one has to do a lot of interpretation just to figure out what the hell the author means. Siegel's book _The Developing Mind_ would have been so much more fun written as an enthusiastic, evangelical, present authorial voice kind of book.
I used to read a bunch of biography-of-a-business/industry books (Uncommon Grounds by Pendergrast was one of the better ones, but there was one about chocolate and others as well). They had this characteristic as well.
I think I really just like to read about other people digging into a subject. Digging into a subject is something I do, something I strongly connect to. How odd that I did not notice this before.