These sort of books are meant to be discursive in nature giving one an exposure to various facets in a domain. That is their "theme". For a layman/common reader this is actually quite welcome since it does not drown-him-in-depth/demotivate-him but gives an overview (possibly interconnecting) of the subjects involved.

For example the Stillwell book i mentioned above has chapters on Arithmetic, Computation, Algebra, Geometry, Calculus, Combinatorics, Probability and Logic thus making it an excellent book to peruse.