It 100% follows the book with the addition of a side quest and new character. Just that you only get Tungdil's point of view, so some characters and events may appear a bit sudden to players unfamiliar with the source material.
The game isn't extremely long, so by the time you are done with the gimmick of horde combat, it may be over anyway.
The book is Heitz's best book in my opinion, though it already has the issue of certain characters being too powerful. This becomes worse in later books in the setting.