I agree with all answers above, made similar experiences editing Audio Books (Hörbücher) here in Germany .
Most studios look for freelance editors that are reliable and deliver their work with consistent quality in the scheduled time. Deadlines are very important because the publishers will only choose studios that can fulfill their schedules. Voice actors and directors are allowed to cause delays but the audio engineers are not!
Editing an Audio Book is a very, hm..., minimalistic task. Most of its challenges are to cut out a hundred little clicks and unwanted noises from the actors mouth. Or to paste together the performance of an inexperienced actor from A LOT of takes.
In my opinion it is also a benefit if you got an interest in literature and the art of reading. If you understand the dynamics and the pace of text, you get a better understanding of the actor's performance. Sometimes you have to choose the best take yourself and correct or create the rythm of speech. Choosing the right lenght for the different pauses is an important part of the job. Sometimes you have to work with a director and most directors apreciate it if you quickly understand their instructions and aid them in their work.
If you have a literary background, (maybe you have, cause you want to edit audio books?) then mention it in your portfolio. This will seperate you from most other audio guys who come from music backgrounds.
Pardon my English and greetings from Berlin!