Skip using rar files and zip files. if you are doing this to be downloaded then use a utility called 7zip this will compress the file to a 7z format with increased compression using LZMA compression. This can decrease the size of a file by around 50%. The problem is most users don't have this utility so you will also have to provide a link to it.
http://download.cnet.com/7-Zip/3000-...html?tag=mncol It is a free utility as well so that makes it more enticing.
Also since this file is just a reading of the book and I can not imagine it having to much music or additional sound effects. Try dropping the quality this could save you file space as well. Even going to a wma file format could help as a MP3 is really based more around a music file so you may get a smaller file this way with a decent sound.
If you are wanting to stream this to your site then maybe the best way to go would be to actually use a wma on a lower quality as the file size would be smaller and will stream faster then a MP3.