How do you read, learn from, and assess a textbook?
Personally, outside of the elementary level I find there are very few (single) textbooks that I can read and understand the subject.  I find it very useful (although cumbersome, and oft expensive) to acquire multiple books on the same subject, so I can read sections from each side by side.  A given author may be better most of the time, but there are enough places when self-studying that I get lost that I need to seek additional explanations or examples.

The opinion of the use of the other books I think comes from answering whether it was it useful for me when learning the subject area.