The book Stay a While and Listen does a good job of explaining the background behind the pitch and subsequent changes that made Diablo what it is today: http://www.amazon.com/Stay-Awhile-Listen-Blizzards-Video-Gam...

You might also enjoy the Blizzard book:

Stay Awhile and Listen: How Two Blizzards Unleashed Diablo and Forged a Video-Game Empire [1]

[1] http://www.amazon.com/Stay-Awhile-Listen-Blizzards-Video-Gam...

Also recommend reading Stay Awhile and Listen: How Two Blizzards Unleashed Diablo and Forged a Video-Game Empire - David L. Craddock http://www.amazon.com/dp/B00G8UL474

Really interesting read, covers Diablo/Warcraft development from its humble beginnings.

There is also a book [1] about Blizzard that I really enjoyed.

[1] http://www.amazon.com/dp/B00G8UL474