There is a dedicated Narrator tag that Audible uses (©nrt) but since not all audiobooks come from Audible, most of the audiobook community uses the composer tag instead for tagging.
Artist/Performer is used for the author because it's the most prominent tag for a person associated with the media, with Composer being the second most commonly viewed tag.
For your own use, you can certainly massage the metadata any way you wish, but if it's something you intend to share (say, recording public domain audiobooks for a project like Librivox or whatever) then you probably want to stick with those community standards. I totally get being OCD and hating that the community standards aren't an actual accurate reflection of the reality of an audiobook, but we're dealing with tagging systems that were created for music and then just sort of spread out to audiobooks.
If you want to cover your bases (i.e. have accurate metadata for your own satisfaction, but also use the community standard metadata for whatever app you're using) I would do it this way:
Author(s) (iTunes / ID3 -- I will try to include options for both 2.4 and 2.3)
TIPL: Author or
IPLS: Author (OCD-version)
First Author (for multi-author books, or editor of an anthology. Whoever you want to be most recognized/searchable for this book)
TMCL: Narrator or
IPLS: Narrator (custom)
Honestly, there are, like, 2-3 different tags that can be used for this.
TPUB are both the most common and the most accurate, but as @ohrenkino pointed out, some apps won't recognize this. If they don't, they're probably looking for
©lab (label -- not sure if there is an ID3 equivalent) or possibly even the studio tag, which I can't remember off the top of my head.
If I had my druthers, I'd actually list individual books in a series the way discs in a multi-disc album are tagged. So "Album" would be the series title, and then "Title" would be the book title, and you'd use the "disc#" tag for the series index. However, I came late to the party wherein the community standards--what little there are for this purpose--were established, so I was not consulted. So there are any number of ways to do this, and none of them are going to be universal. Here are a couple, however:
series-part for this purpose (I don't remember off the top of my head what the MP4 code is for these, but I think it's one that Audible created and not part of the standard library).
I think that tag probably gets lost when converting to ID3 though, so you could create
TXXX: Series and
TXXX: Series Index tags for this. That's one option.
Some people use the Grouping or Content Group tags for this:
TIT1 - Series title
A more precise way to go is to use the movement/movement index tags:
MNVN - Series title
MVIN - Series index
That seems to be the second-most popular option, probably because those tags already exist in both MP4/Quicktime tagging and ID3 tagging.
If you have books that are in a multi-series universe (eg. Terry Brooks' Shannara universe, you could also use the "Work" tags in addition to the "Movement" tags:
TIT1 - Universe title
Since I prefer to use .m4b files rather than .mp3, I like to go with the tags available for TV Shows:
tvsh - series title
tven - book title
tves - series index
tvnn - Universe title
tvsn - Universe index (i.e. where the sub-series falls within the universe as a whole, so you know which subseries to read in which order.)
There's no real advantage either way, since I don't believe most apps look for series data (at most, they look for the subtitle so they can display "Series Title, Book Index")