Listening to MP3 audio books in iTunes and your iPod is troublesome. They show up as regular songs and not in the special "Audio Book" category, thus options like changing speed or remembering playback position are not available.
You can find many tutorials about converting MP3s to Audiobooks. Most of them tell you to join multiple MP3s to one file, convert this file to .AAC in iTunes, rename its file extension and readd it to the iTunes Library. That might work well, but you might also use the free and Open Source tool Audiobook Maker.