MyAnonaMouse (MAM) is a great private tracker dedicated to audiobooks, ebooks, sheet music collections, instruction media, Guitar/Bass Tabs, Lick Library and radio shows.

Enabled Users 65,430
Torrents 690,423

Maintaing a good ratio is easy, as you will receive a lot of bonus points once you join. Not just that, there're a lot of freeleech torrents. Better yet, once you become a power-user (after 4 weeks of being a member), you can purchase a VIP class (with the bonus points) that will give you access to a huge selection of VIP-only, freeleech torrents.
The content and the overall speed is also considered very good, you can find almost any audiobooks, ebook etc on this tracker.