Aside from biographies, which are plenty and mostly good, there's nothing better than being able to go to the source! All of his surviving notes are available in illustrated and annotated forms.

I found them once when I was at the uni library... before I knew it hours had passed and I was still cross-legged on the floor going through them.

Also "Leonardo's Universe" is great to have... full of illustrations and notes about his time.

My favorite is called "How to think like Leonardo da Vinci" http://www.amazon.com/How-Think-Like-Leonardo-Vinci/dp/04405... It also has a companion workbook. Who wouldn't want to think like Leonardo !