Is it counterproductive to read very long text books during an MSc program?

I haven't heard of such a thing, but can understand where it comes from. But my advice would be that doing teaches you much more than reading. In CS we often build things to reinforce learning. Textbooks can provide useful exercises. And one learns, primarily, through repetition (reinforcement) and feedback. Only a few people have a "photographic memory" and even then, being able to recall some text doesn't mean that you have gained insight from it.

Seek insight. Reinforcement and feedback. Hopefully the professor provides feedback, but programs (when they fail) can also do that.

I don't think the professor means to ignore textbooks, however. Use them "just enough" to gain insight.

That's ridiculous advice. Believe it or not, lots of thick assigned textbooks contain useful information that's not covered in lecture.

It comes down to whether you want to learn more things or to pass the course (note they are not synonymous).

• If the first, then no, it's not counter-productive. You will learn more by reading the textbook, almost by definition.

• If it's the second however, then yes, it's counter-productive. It's the material covered in the course that will actually end up on the exam. Accordingly you want to study that material and that material only. Any time spent on things that aren't in the lectures is time wasted, and chances are when you read these supplemental textbooks, much (or even most) of the material will not be covered.

Unless you spend significantly more time on the course, viewed another way, these two choices basically come down to "do you want to know more things" vs. "do you want to master a few things". It's not obvious which is better.

Finally, do note that although the first option might sound better, in practice it's usually the 2nd that matters - in a job interview or PhD application, the grades on your transcript are important, and if those are poor you will often not get the chance to demonstrate that you know more than the grades indicate.