I had the same problem as the rest of you, here is the answer,
I also receieved an email from apple (iTunes Store <do_not_reply@apple.com>) here,
Dear xxxxx,
You have successfully created an iTunes Connect Free Books Account. To complete the process, activate your account.
After you've activated your account, go to the Deliver Your Content module, download iTunes Producer (requires Mac OS X v10.6 or later), and follow the instructions to submit your book to iBooks.
A copy of your contract is attached to this email for your reference.
Regards,
The iBooks team
After downloading iBooks Author I could not complete the log in process within the App either,
I therefore did a little research and logged an issue with ibooks_applicationsupport@apple.com noting the same error message as the rest of the thread, although I could also have called 0800 975 0615 to achieve the same outcome.
I received the following email,
Hello xxxxx,
Thank you for contacting iBooks Publisher Support regarding iTunes Connect.
We reissued your invitation to iTunes Connect. Please check your email for this notice. If you do not find this notice in your inbox, we suggest checking your spam filters. You'll be able to click through this email to sign in to your new Books account and complete the contract process.
If you need further assistance with signing in to iTunes Connect to activate your Books account, please reply to this email or call us directly at 0800 975 0615. Support hours of operation are 7:00 a.m. to 5:00 p.m. Pacific standard time, Monday through Friday.
Best regards,
Julie
iTunes Provider Support
This then triggered the missing email from apple (iTunes Store <do_not_reply@apple.com>) here,
Dear xxxxx,
Your application to distribute Free Book content on the iTunes Store has been approved. Next steps include agreeing to contracts and delivering your content. Visit iTunes Connect to set up your account or sign in to get started.
Regards,
The iTunes Store Team
I could then then log into this site,
https://itunesconnect.apple.com/WebObjects/iTunesConnect.woa/wo/4.0
To then click on "Deliver Your Content" and jump to the download link,
https://itunesconnect.apple.com/WebObjects/iTunesConnect.woa/wo/6.0.15.25.0.0.1. 3
Where you can download,
iBooks Author
iTunes Producer 3.0 (or 2.9.3 as I did)
iBook Proofer 1.0.1
Featured Resources
Additional Resources
I then downloaded "iTunes Producer" and "Book Proofer"
And signed into iTunes Connect in "iTunes Producer"
And then signed into iTunes Connect in "iBooks Author"
(I must admit that I did have to quit iBooks and re-launch it to get this part to work whereupon the App no longer prompted to download iTunes Producer and logged straight in to iTunes Connect).
Hope that helps everyone. 😝