There may not seem to be anything special about the math textbook you use, but there are actually some texts considered by the mathematical community to be classics.  They're usually considered exceptional because of how clearly or thoroughly they're written, or because they were authored by the mathematicians themselves who were fundamental in starting that particular field of math. 

Math textbooks definitely are written in a style that reflects the historical time period.  Math vocab words come in and out of style, and proof methods or the use (or lack!) of figures in the text changes over the years.  Some of the older texts are hard to learn from at first, and it might be helpful to use them as a second textbook for a particular math field.  Others have been revised and updated several times and are more readable.

"Classic" is a subjective term that, in this list, is mixed a little with "recommended."  Opinion is definitely involved when making "classic" lists!