Nice article. Lots of examples of terrible math books can be found at

http://www.mathematicallycorrect.com

My favorite is from the Connected Mathematics Program, one of 5 math programs the U.S. Dept. of Education identified as "exemplary":

A quiz for seventh-grade CMP students asks them to find the "slope" and "y-intercept" of the equation 10 = x - 2.5, and the teacher's manual explains that this equation is a special case of the linear equation y = x - 2.5, when y = 10, and concludes that the slope is therefore 1 and the y-intercept is -2.5.

The corect answer is that the slope is infinite and there is no y-intercept, since the line is parallel to the y-axis. This is so elememtary, that the author of the quiz obviously has no understanding of mathematics.

You would think that since this program is "exemplary", there must have been a lot of people looking at the textbooks. Apparently none of them noticed the error, or thought it was bad enough to knock the program out of the "exemplary" category.

Jim