Neat little tip...thanks!
The simple trick? Just right-click a track and select Get Info, head to the Options tab, and then select Audiobook from the Media Kind drop-down menu. The file will instantly leave your Music library and head straight for your Audiobook library. To mark multiple files at once, just select them all and go through the same process. The only remaining step is to tick the Remember Position checkbox if you haven't already, and your tracks should now have easily found their way to your Audiobooks section, and even better, they should work like an audiobook.