Well, it depends on what your intended use is. If you simply want smaller files for periodic listening, you could go as low as 24kbps...but that will give you the quality of an old AM radio. Longer listening would likely drive you nuts.
I would stick to the 128kbps neighborhood...not much lower than that. 96kbps would also be decent for spoken word content.
May we assume you already know how you will end up ripping them from tape? You have software?
motoyen wrote: I want to rip some book I have on tape into mp3 format. Any suggestions on what bitrate I should use that will allow for decent spoken sound and small file size?
For that sort of application, anything above 128kbps will be overkill. I'd wager somewhere around 64-96kbps should be enough, but everyone has different tolerances when it comes to bitrates. What you should do is take a few minutes of audio, encode it at different bitrates, and see what the results sound like.