I’d start with Vasari’s life of Leonardo. You can find English translations online. I have a good biography of Leonardo at home, but can’t remember the author’s name right now. I will try to get that for you this evening.
On the Medici, Christopher Hibbert is pretty good: http://www.amazon.com/gp/product/0688053394/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0688053394&linkCode=as2&tag=thewesttrad-20&linkId=NKUZD32CASSO7GNU