People say „you can’t be too careful“ all the time.It‘s extremely common. And, it seem to emphasis to being careful. Like the others pointed out there’s a grammatical error in the book you quoted. It should be „in preparing“ not „to prepare.“

Your book has an incorrect sentence in it. The verb should be in the gerund form, not the infinitive. It should be: "We cannot be too careful (in/when/while) *preparing* this presentation". For the meaning, look at these examples: You are too careful. = you use too much care You can be too careful = you can use too much care You cannot be too careful = it is not possible to use too much care / You cannot use too much care So, the sentence means "It is not possible that, when preparing this presentation, we use too much care." or "We can't overdo being careful when we prepare this presentation."

Yes, you are correct. It is a strong way of saying "we must be careful." We must be very careful. It is not possible for us to be too careful. The more careful we are, the better. "We cannot be too careful" is, indeed, hard to understand. You don't need to use this form yourself. You should understand it, but when you are speaking or writing it is fine to say "We must be very careful..." (I don't actually think the sentence you copied is perfectly correct. I would write "We cannot be too careful in preparing this presentation." I'm not sure. I would like to see what others think about that).