I am not sure if I can answer this without my comment being removed since it is based on my opinion and not on a peer reviewed research, but as a person who deals with Italian art and literature on daily basis, the best literature pointers for da Vinci's life and work can be found here

André Chastel's Léonardo de Vinci par lui-même, is also very, very good.

If you want a biography as close to the times of Leonardo as possible, Vasari is always a good choice, and I believe his work is in public domain, too.

Now, it goes without saying that these are just my opinions and that other scholars and students may find other sources more useful than the ones I have listed here.

N.B. I do not know how many, if any of these works are available in English.