Nick Drake: The Biography by Patrick Humphries
However the book does suffer a bit from being repetitive in places and consequently a bit overlong - for example the middle section labours many examples of Drake's discomfort on stage, and is sometimes disjointed giving the impression that different sections were written at different times out of order and then joined together without an overall edit. For instance established characters are (re)introduced later in the book as if for the first time, and vice versa.
These criticisms aside the book did encourage me to dust off my Nick Drake CDs and listen to them again with renewed insight.