Are you rooted or not? If you are,
Adaway is a good choice. If not, you could try
Adguard. I haven't tried Adguard myself, as I am rooted. You could even install the ad blocker just long enough to get FBReader Bookshelf to run and then purchase the Bookshelf and remove the ad blocker afterwards. I don't know if either ad blocker, however, will work on Android 2.3.