What is the font used for the cover of James Stewart's Calculus textbooks?

Since I can't locate this book to evaluate it, it would be hard to tell.

You can try using a font library, many of which are available online, that shows samples of all the various fonts out there, both common and specialty fonts. You would have to study the various details of the font on the cover to find the right match, including, but not limited to: serif or sans, design of specific characters (letters), length or curvature or decoration of stems on characters, etc.

It's even possible that it may be a custom-made font just for that book cover. Many times, a graphic designer will take a base font, convert it into individual vector drawings, and then alter it in various ways even to the point that it no longer directly resembles the original font. Or it may actually be a font that was created by the company's designer that is directly type-able straight from the keyboard, and they are probably the only ones who actually have that font file.