Leonardo has been mythologized and then de-mythologized many times. He is worshipped and he falls into disfavor. His greatness may not be as much as we (and he) want it to be, but his drawings still hold a power. Out there there are lots of lifelike works and yet they remain lifeless, but Leo's sketches, for me, always seem to be able to capture the very essence, the vivacity of their subjects.
This is an old book but it still serves well as a great introduction to Leo's artistic life. Clark states carefully that he does not aim to explore Leo's achievements as a scientist, but he does mention briefly what Leo was involved with and his ambitions as well as frustrations.
Martin Kemp's introduction is particularly valuable.