The idea is certainly grand. However, there are several problems that should be considered.
1. As already mentioned, such a book could only be aimed at HoMM fans who are familiar with the history and laws of this world; it's simply impossible to explain all of this to an outsider—the plot would get lost behind endless author's notes. But there aren't that many of us; personally, I've met 2 of our own in real life, and even they were former fans. Publishers won't bother with a book that has such a small target audience.:rolleyes:
2. A book cannot create plot interactivity, which is exactly what many of us value.:smile09:
3. The story of each part (especially 5) is quite dense, making it problematic to integrate a major saga; and the transitional periods are too short in time to be used.:smile38:

And if brave heroes actually decide to write it, I would recommend making a story about Agrail's youth and how an Elf happened to become the Demon Lord.:smile30: That would turn out interesting.