What are the most frequently cited textbooks in the physics literature?
The answer you might be looking for lies in this recent Nature News article on the most cited scholarly works of all time.

Specifically, for evidence supported by real data I'd look at the Google Scholar top 100 spreadsheet file

In this, you'll find Google Scholar citation data for physics-related books like: 


Principles of optics: electromagnetic theory of propagation, interference and diffraction of light (1959) with 49132 citations
Classical electrodynamics (1962) with 36945 citations
Physics of semiconductor devices (1981) with 33789 citations


It depends what you define as a 'textbook' though, I suppose...