I would assume an audio book editing/producing not to generally be a specific specialism, but something that could be worked out in any audio studio by anyone who otherwise works in audio production for media. It isn't different than other forms of audio production that isn't strictly music production or different from other types of "sound design", even though audio books are a slightly different type of medium from the others. The production process is still similar enough.
I would assume that an internship in a sound or broadcasting studio that produces audio books would be a good call. I doubt book publishers have in-house audio studios, but rather they outsource the production of an audio book to a sound studio or a broadcasting company. It might be also difficult to freelance in just audio books, because I don't know if audio books are produced on an "indie" basis that much, so it could be difficult/impossible to find projects that way.
If an internship is not an option (at least not immediately), then if audio books are your sole interest, then I'd probably try to work out some small demos, for example based on existing books or short stories or even scripts. I.e. just producing some audio book sound by yourself, so you can have something to show to prospective employers/customers/affiliates. That's the current standard procedure in the media and entertainment field anyways, building a portfolio of work that applies to the line of work that you're applying for and getting someone interested about it.