Why not get a program and encode a section at various bitrates, using each
format and use *your ears* to decide if the quality is acceptable to *you*?

In general , for similar quality a wma file is around half of the size of an
mp3 for similar output (for music). As speech does not have the dynamic
range that music does start at the lower bitrates that your device and
selected program(s) and work up.

Remember it is your ears that will be listening to the books, use them to
decide the bitrate that is acceptable to you.

Personally I find wma are better from a file size point of view (see above),
but remember that your chosen player must support the bitrate too- I found
my intended players bitrate range did not match that of the encoder.
(eg 8kbps, 8 kHz, mono, CBR
112kbps, 8 kHz stereo CBR
were the lowest supported by the encoder but were not supported by my
listening device).

Variable bitrate encoding may help final size as spoken files can be
compressed a lot, but I found fixed rate was better here for the devices I
used because of final file size even using lowest quality settings- it
seemed designed for music.

Why not try encoding at the lowest rates supported by your player and
listen? If it sounds ok then all you have to do is choose the smaller
filesized format, if it doesn't sound ok move up the range..

I have spent many hours playing with books on CD and
moving them to audio files. I have hundreds of hours of
audio books now in wma format. here is my two cents.

Use Windows Media Player to rip the CD to the hard
drive.  This will make wma files, but might not allow you
to make the files as small as you want. (smaller file =
smaller bit rate = less quality)  In WMP 9 go to "tools"

where ripped files are saved, and the bit rate.  The
lowest WMP 9 will allow is 48 Kbps or 22Megs per CD.

If you have Windows XP use Movie Maker 2 (free update
from Windows update site)  I know this sounds a little
odd, but if you add only audio files to your "movie" MM2
will encode the final product as a Windows Media Audio
(wma) file.  When creating your "movie" in the "movie
settings" part of the "save movie wizard" you can choose
your bit rate.  Remember if your source files from WMP
are 48Kbps than there is no need to experiment with
anything higher that 48Kbps.  The advantages of taking
the second step and using MM2 are listed below.

1.  You can easily trim off intros or endings

2.  You can add multiple tracks together and create one
file

3.  Bit rates as low as 8Kbps designed for voice. 1 CD
(77min) at this rate is ~4.5Megs as apposed to 22Megs
using WMP.

4.  You can choose bit rates of 8, 16 or 32Kbps.  Start
with 8 if the quality is acceptable you don't have to go
any further.

5.  You can add properties to your files like author,
title, description.

One drawback for using MM2.
These files can only be played on devices that are
compatible with the Windows Audio Codec 9.  Windows
Mobile 2003 for pocket pc and newer portable players can
play these files.  Almost all computers can play these
files even if they are using an older version of WMP by
automatically downloading a codec from Microsoft when the
file is opened.

I am glad to answer any questions,
Matthew