I admitted it might be a reach, and I realize that many books begin with a flashback or preview of something that will follow in more depth, but in Wilson's book, I had the distinct impression that the original book did not include the Niger material. There was really NOTHING about it in the proper chronological spot, only at the beginning of the book. I'd be very curious if some earlier form of that book had been previously submitted to a publisher and rejected; the inserted Niger material would have given it marketability it didn't previously have.
Actually, I think your conclusions may be right. If I recall, Wilson had a couple months to criticize Bush's SOTU and didn't do so. In fact, there are a number of public statements that Wilson made directly after the SOTU address and Wilson doesn't even comment on the 16 Words. One of those appearances is even documented here on FR with Bill Moyers (2/28/03).
From what I remember, not only does Wilson not seem to show any interest in the 16 words, he agreed with much of the speech and emphasized the danger that Saddam posed. I also remember a Nation piece he wrote in March, that while a little more critical, still mentioned nothing agout Niger and Uranium. It wasn't until May of 2003, just before he officially joins the Kerry campaign, that he has a change of heart.
Oh, I'll bet he was working on a book...his ego is so big, I'm sure he thought (mistakenly) people would be interested. And then insert the only chapter people are really interested in at the beginning of the book.