Question: It seems pretty normal to see screenshots of MS Word, steps for "this is how you generate a template". I don't think Microsoft would object.

But I'm not sure about reprinting images from in-game scenes for specific games, like Diablo II. Do you need to obtain permission to include a screenshot of a game in your book?

Answer: 
There are four factors to determining Fair Use of copyrighted material that must be weighed in the balance:

* Purpose and character of use...better if not for profit; better if transforming in some way
* The nature of the work...factual is better, creative works less so
* The amount and substance of the work...less is better; uses that don't get at "the heart of the work" are better. There are no rules about percentages, etc, despite myths to the contrary.
* The effect of the use on market value...if it doesn't take away from real or potential profit, better

Depending on the nature of the book you are writing, you are probably quite safe using screenshots. You are likely writing something that will stimulate sales and that isn't going to reveal workings of the game that are protected.

Your publisher should have experience with this and probably has a policy for when they ask permission. Unfortunately, many people and organizations don't understand Fair Use and err too far on the side of caution, ceding rights they don't actually have to.

I teach about Fair Use regularly; you can find more information here: <http://iteachu.uaf.edu/develop-courses/constructing-a-course/copyright/>
There is a legal consideration concerning using part of one copyright work in another. The rules around it involve the quantity of material copied and the purpose for which it used.

However in your case there may be an over-riding consideration. If *Blizzard Entertainment* took exception to your use of material from *Diablo II*, would it harm the market for your book?

If there could be a negative impact from not seeking approval, then ask, whether or not you are strictly required to by law.