Nokia Audiobooks is a rather excellent system that makes it much easier to listen to audiobooks on your S60 phone.
Nokia Audiobooks uses the ABR-WB audio format (optimised for high compression of voice). This allows the files to be encoded at a lower bitrate resulting in much smaller files. A typical audio book will be compressed as an ABR-WB file at 12.65kbps compared to MP3 at 64 kbps, representing a 500% space saving. For example volume 1 of Aesop's fables comes in at 22MB for MP3 but 4.3 MB for ABR-WB.
There are a number of places where you can get public domain audiobooks (e.g. Librivox [more than 1000] and Podiobooks). However it is possible to covert commercial audiobooks provided on CD (though please check the legal situation in your country). As noted on the Nokia website it may be possible to borrow audio books from your local library.
The Nokia Audiobook Player allows you to play back ABR-WB files, but also offers a number of features designed to optimise audio book playback. These include a book 'browser' which allows you to choose which book (and which chapter) to play back. There is also excellent support for bookmarks and the player will auto-save the last position (meaning the player will start from the same place in the same book when you next start it). There is also a sleep pause function which stops the playback at the end of the current chapter (useful if you are listening as you fall asleep). Power uses will appreciate the built-in keyboard shortcuts. The developers deserve credit for an application which is well though out and easy to use.
Nokia Audiobook Manager, A Windows PC program, acts as a companion application to Nokia Audiobook Player. It enables you to convert existing files (WAV or MP3) or audiobook CDs into the optimised format and then copy them to the phone for use in the Nokia Audiobook Player. While all S60 phones can play MP3 files converting the files not only saves space, but allows you to use the audio book optimised player (e.g. bookmarking).
Features of Nokia Audiobook Manager include the conversion of WAV or MP3 (if Lame encoder installed) files to ABR-WB format, selectable bit rates in expert mode, the ability to extract audio from a CD and a copy files to phone function (via the USB Mass Storage connection option built into most S60 phones). All audio books are stored in the Audiobooks directory in the root of either phone or card memory (C:/Audiobooks or E:/Audiobooks).
See the Nokia Audiobook website where you can find all the necessary software, information and a few sample audiobooks.