How can I get tracks from another iTunes library onto my iPhone without wiping it?
GoodReader for iPhone ($5) will let you load files directly to your iPhone over your LAN and play them from there. That should work fine if your audiobooks are a) all one file (if they are in multiple files there may not be 'playlist' support, which would be annoying, and b) not DRM'd. If they are DRM'd you'll need to sync them via iTunes or, in the case of Audible.com, you can download them from the Audible player.