Most of the small books I'm familiar with are math books. For example, a few on distributions and Fourier analysis are
Lighthill - Fourier analysis and generalized functions
Richards and Yoon - Theory of Distributions
Strichartz - a guide to distribution theory and Fourier transforms
For physics, one small (but quite specialized) book I enjoyed was,
MacDonald - Noise and Fluctuations (~120 pages, undergrad level)
I'm not sure if it is too big, but I also really like
Kittel - elementary statistical physics
which has many short chapters (I think each is one lecture) so is pretty digestible in small chunks. It is longer (> 200 pages), but the old hardback I have is remarkably compact and I'm guessing the Dover paperback is pretty small as well. I only read the last half of the book which covers noise, fluctuations, non-equilibrium situations and kinetic theory. It says 'elementary' in the title, but I believe it was for a graduate-level course taught in the 1950s.
A quirky text written for an undergrad mathematical physics course at University of Iceland is,
Jonsson and Yngvason - Waves and Distributions
It starts with continuum physics and properties of plane-waves, but for me the core of the book is chapters 4-6. Those cover distribution theory and Fourier analysis, then use those tools to study properties of waves (Green's function and radiation, multipole expansions, Huygens principle, mean-value theorems, etc). The best chapter is the last one on waves in dispersive media, and I wish I had read it prior to taking graduate courses in plasma physics. It doesn't go into all of the pre-cursor physics, though. For that there is another small book,
Brillouin - waves and group velocity
The first few chapters my fit the bill, but if I recall correctly some of the later chapters dive deep into things like asymptotic expansions of integrals with poles near saddle-points and such, so are probably not what you would want to read in 15 minutes on the bus...
jason