The following is an abbreviated list of "books" (the ancients used this term to refer to what we would now call a lengthy treatise) of Aristotle that we don't have. The original list was compiled by Diogenes Laertius - not always the most reliable biographer - but it's just about all we have. Imagine getting your hands on one of these:
Of Household Management
Of Friendship
Of Prayer
Of Wealth
Concerning Love
On Sterility
On Noble Birth
Prognostics of Storms
On Music
On the Animals of Fable
Proverbs
Letters to Alexander
Then there are Aristotle's Dialogues that were so highly praised by Cicero. In this vein, I recommend Richard Rubenstein's recent book Aristotle's Children. Among other things, he discusses the rediscovery of Aristotle's works by Western Europe via the great School of Translation in Toledo and the impact they had on the Catholic Church and the rise of the great European universities, especially the University of Paris.