Why do I need to make the tracks bookmarkable?
From what I’ve read and experienced, only two types of files are truly bookmarkable: Audible.com’s proprietary format, which iTunes can play, but not create; and AAC files which iTunes and iPods believe are in the .m4b Protected AAC format. MP3 files can be made bookmarkable, but do not show up in the Audiobooks section of an iPod, so I’ve stayed away. Note that an iPod will normally hold your place on a given track, even across off/on cycles, but this is not the same as being bookmarkable, because it will lose that place on a track the moment you play a different track. This is probably fine behavior when you’re listening to music; you can just play something else. But it doesn’t work so well for the linear narrative structure of most books. So being bookmarkable is very beneficial if you use your iPod for audiobooks (and podcasts, but that’s another subject).
