iBooks Author accounts are free, existing developers need a new account
Setting up an account to publish books to the iBookstore has always been free, and the new iBooks Author tool has brought that fact into the spotlight. Self-publishing books to the iBookstore does still have a few hurdles you have to leap through, however. As AppleInsider points out, for iOS and Mac developers in particular, one hurdle is that existing iTunes Connect App Store accounts won't allow you to upload books to the iBookstore; instead, you'll need to set up a separate Apple ID associated with iBooks Author.
Account setup also requires you to input credit card information and have a US-based tax ID -- for individuals self-publishing content, a Social Security number will suffice. For some odd reason, iBooks also require an ISBN (a requirement the Kindle Store doesn't have), and those aren't cheap. In the US, purchasing a single ISBN from Bowker costs $125, or you can buy them in discounted packs. Personally, I have no idea why they're allowed to get away with those prices; ISBNs in my country are available for free.
Speaking from personal experience, setting up a publisher account on the iBookstore is a lengthy and somewhat unintuitive process -- but nowhere near as slow and frustrating as dealing with traditional publishers. Developers not being able to use an existing iTunes Connect account to publish iBookstore content is a bit baffling, however, and hopefully it's something Apple will address.