iTunes had stored some of my audiobooks under Music instead of Books. Actually, I had not realized that these were mutually exclusive lists under I wanted to transfer some audiobooks to an iPhone and realized certain titled did not appear under the Books tab.
All this time, I had been flagging titles as Audiobooks under Genre, not realizing that I needed to change additional settings to move them over to Books. First off, if your audiobooks are in MP3 format, you should convert them to AAC so that iTunes can remember the playback position. In the Music tab, you will be able to see all versions of your audiobooks. However, under the Books tab, I think iTunes consolidates the listing so that if you have both MP3 and AAC versions, you will have a difficult time deleting the MP3 version. Before you convert the MP3 to AAC, remember to visit iTunes > Preferences and adjust your Import Settings under the General tab. If you are regularly encoding as iTunes Plus, your AAC audiobook will end up much larger than the original MP3. So, selecting Spoken Podcast under Import Settings fixes this problem.
Next, select the AAC version of the audiobook press command + i to summon the information window. Under the Options tab, selecting Audiobook under Media Kind will then move the audiobook from Music to Books.