If you want good textbooks, make sure the author is trying to persuade the students of something, I'd say. I usually am.

Perhaps the process of writing should be separated from the product of writing (i.e. the textbook). The best of both worlds surely is a textbook that doesn't try to persuade at all (since persuasion is tangential to providing an explanation), but which was written with a process involving a lot of arguing (to help stimulate the best reasoning). My brother and I sometimes had heated arguments when we wrote C# 3.0 in a Nutshell, with numerous "red ink revisions" before finally settling on the NPOVish text the reader sees.