[ProDigit]

One of the reasons why OGG is 'less known' than AAC, is because OGG came in a time where DRM audio became everything!
AAC supported DRM'ed 'encryption', while OGG did not.
So most manufacturers went for AAC. AAC is very good, but it's compression ratio is not so very good. In the end customers hardly ever used DRMmed audio in their devices, perhaps just like here the case with ebooks, and got off AAC. That's where I have the impression (between 2002-now) that AAC has been in a dark corner somewhere, rotting for a couple of years... Sure, some devices had the playback ability, like the PSP, but I don't think I ever used an AAC audio file, except perhaps if they where included in the PSP games.

Implementing OGG in a device costs less to the manufacturer than implementing AAC.

OGG is found more often in encoded movies (OGV, and even more in MKV containers).
AAc can be found in Mpeg4 AVC encoded videos. AVC (or H.264/MPEG-4) is one of the better video encoders out there (perhaps even better than Xvid or DivX).

But I'm less fond of the audio.

I also bought a Cowon D2 for the simple reason that it was one of the first touchscreen DAP's that could play back OGG, back in 2005.
Likewise in 2000 I bought one of the first CD DiscMans that could play back lower bitrate WMA data cd's from Panasonic.
I didn't even knew that my D2 supported AAC (with a firmware upgrade); I just didn't find the free encoder tools to encode audio neither the files to that format online (like I found from OGG).

I think people who prefer to use Windows will prefer to use ogg.
Most of the (few) .m4a files I have on my HD (supposedly a sort of aac encoded files) I reconverted to OGG for saving disk space.

I only push OGG because I work with it day to day.
On my windows PC, I only get in touch with AAC very infrequently (like when downloading a sample file from a MAC freak website or so...).

For audio books, WMA does make a lot of sense, because of it's internal harmonic booster, which will make lower bitrates (sub 48kbits) still sound very clear, and bright.
Sub 48kbits with good audio I have not yet seen in AAC, MP3, nor OGG formats.

.WMA 9 is an old codec, but it should not be underestimated in the lower bitrates.
For higher bitrates than 128kbits, I'd recommend another encoder than WMA.