The ERA, who organise the Rally, publish a book after each event. The 2013 edition has great pictures but the text reads like a shopping list of which car is doing what.
Prince Borghese's Trail by Genevieve Obert follows the Nepal/Pakistan/Iraq route that is now impossible due to various conflicts along the way.
Allen Andrews' book The Mad Motorists is about the original 1907 race. You might prefer this to the Luigi Barzini book mentioned above if you prefer a more modern writing style.
How to Build a Successful Low Cost Rally Car is by Philip Young who founded the ERA and died last year. Personally I think it's bonkers to imagine you can undertake P to P in a really low cost car but there are some great and practical tips in this book.
How to Win a Marathon Road Rally is published by the ERA and written by Alan Smith who is one of those lovely low key people you meet who just quietly know their stuff. I keep a copy in my navigator's bag.