A large audio book is here defined as one that needs chapters and sub-chapters to navigate meaningfully. For example, an audio Bible. It would have 1189 chapters when using one navigation level alone, which is clearly unnavigable.
There are two alternatives in principle. The first is to store each chapter as a separate audio book, so that each subchapter becomes a chapter of such a separate audio book. This is simple to achieve: just create one folder for each chapter in the AUDIOBOOKS/ folder of the Sansa Clip. This gives you one audiobook per chapter, where you can then place the files (one per subchapter). The problem with this approach is that the player will repeat a chapter (as it thinks it is a full audiobook) instead of going to the next one.
The second approach is to store the audiobook in the MUSIC/ folder, one folder per chapter, one MP3 file per subchapter. And then to create playlists, one per chapter, named by chapter, starting with the first subchapter of that chapter, and going through all the subchapters sequentially, ending with the one immediately before the first one in the playlist. That way there is one playlist per chapter (allowing to jump quickly into each chapter), and then one can navigate within the playlist form file to file between the subchapters. In addition to the first approach, the playback does not stop and restart at the chapter boundaries, but moves on to the next chapter, repeating the whole playlist (that is, the whole audiobook).
In the playlists, you can use the #EXTINF section of the M3U playlists to give the real
name of the chapter instead of the filename. [source]
You can also combine both approaches: since firmware 01.01.32 resp. 02.01.32, the bug “Playlist referencing content in Audiobook folder does not work” is removed, so you can have playlists referencing audio books. [source]