During a completely unrelated conversation I was having with a colleague a few months ago, they happened to mention that they had just read a book called Digital Identity by Phil Windley and they highly recommended it.
Well, not being one to ignore recommendations, I bought a copy and have just finished reading it. The book is a good guide for anyone working within the 'Identity' space and provides some very good insights from Phil's experiences. In particular, I found the framework proposed for defining an Identity Management Architecture very useful. There are some areas where the book gives a good insight but leaves you needing to go and research topics further. However, all the major components of digital identity and covered and the links between them defined.
This book should certainly sit on the bookshelf of any identity management consultant.