Reading popular books on the same subject have sharply decreasing marginal utility

I think there are so many separate subjects (given that you're reading for fun, not for specific knowledge), that this really isn't a problem.

What you said seems true for for sciences with well-ordered master theories. These obviously include physics, and to a lesser degree perhaps biology, astronomy or comp-sci. Once you've read a few really good books about popular cellular biology or popular evolution, there may not be much more to be gained at the popular level.

But in some areas of study, there are very many subjects which must popularized separately. History is the most obvious one - I've read dozens of popular history books, and will gladly read dozens more, because they're dealing with different times and places, or with different themes such as economic history. Another example is zoology - the biology and ecology of particular organisms.