Well let Brian say this in his own words:
https://www.kickstarter.com/projects/1462758959/commodore-the-amiga-years-book/posts/1813447"Just now, at 11:51 pm, I finished the last of the actual writing for Commodore: The Amiga Years. In other words, there is now a complete book that can be read from start (1982) to end (1994) telling the complete Commodore-Amiga story." and
"The next milestone is a release of the full Kickstarter edition of this book--probably in the 850 to 900 page range...."
As a higher tier backer I just wanted one book to tell the story of Commodore: The Amiga Years. Looks like he finished it, was going to be available to Kickstarters as a book in the 850-900 range, but instead decided to profit further by releasing the book in two parts.
Once again in Brian's words
https://www.kickstarter.com/projects/1462758959/commodore-the-amiga-years-book/posts/1888987"The Amiga Years is currently at 567 pages after the split, and will have to be reduced to a little over 500 pages due to front material, table of contents, index and other things. Ill need to cut around 1-2 pages per chapter in order to make that goal."
So he finished the complete story (in his words) in 850 to 900 words, then decided to split the book in two (at around 500 pages each) so he could sell the other half to us