O’Halloran, K. L. (2005), Mathematical Discourse: Language, Symbolism And Visual Images. Continuum International Publishing Group.
She is a semioticist.
Now another new book also looks at math from a linguistic point of view, this time in connection with how mathematical discourse works in other languages:
Barton, Bill (2009), The Language of Mathematics: Telling Mathematical Tales. Springer.
It is described in Reidar Mosvold’s math ed blog.
By the way, lots of books and articles that use phrases such as “the language of math” are not written from a linguistics point of view at all.