Error 404 on some pages and some posts

OK, I’ve scoured the posts and web for some help here for the last few days with no real progress or resolution. Happy to be redirected to a solution post if appropriate…

The problem?
http://thesuccesszone.com/book is showing a 404 page.
I’m also getting 404s on http://thesuccesszone.com/favicon.ico and various other blog posts (though these 404s seem not to be as permanent as the above page).

I’ve tried:
-resetting the permalinks

-turning off all plugins, reactivating one by one…

-changing the blog address URL in general settings to http://thesuccesszone.com/blog (this allows the Book page to show under the URL http://thesuccesszone.com/blog/book)

-dropping the /book/ page under any other page as a child page (this solves the problem in terms of removing the 404, but drops the Book page off the top menu)

I just can’t make any sense of (a) how this appeared in the first instance (given all was working fine before, no known change) and (b) how to fix the damn problem.

Any help will be received with lots of genuflection and appreciation…

Best regards,
Andrew