How do you read, learn from, and assess a textbook? This seems like a vague question, but please bear with me before voting to close it:

Often times, when searching for a good textbook on a particular subject (mostly mathematics or physics), I see users who confidently suggest multiple textbooks on this same subject. (Here is a good example.) My guess is that these users have not actually read through each of these textbooks page by page.

I'm currently going through a few textbooks to learn and/or brush up on different subjects. I am reading them carefully, doing the exercises, taking notes on things I don't understand, etc. A chatper, at best, takes me a couple of weeks (and I spend the majority of my free time studying). 

This being said, how does a person read these text-books in-depth enough to have a founded opinion on their contents? Is it done after already knowing the material? 

(I know this is a strange, perhaps difficult to answer question, and I'd be glad to take any suggestions or criticisms to better it).
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.