There's a a very good book called 'Pegasus - The Heart of the Harrier' by Andrew Dow with the foreword written by John Farley (ISBN-13: 978-1848840423)
It gives an insight not only to the development of the engine (all variants) and the airframe, but also a history on VSTOL and the UK aviation industry at the time, especially between Bristol and Rolls Royce engine divisions.