There are so many out there. Have a browse on Amazon and read reviews for some before choosing.
Ones I have read include:
Patrick Holford GL -Diet Bible which is an indepth read.
Nigel Denby -GL diet made Easy which is good
Anthony Worral Thompson -GL Diet made Simple which has lovely recipes.
GL Diet Counter by Dr Winnie Chan is invaluable for checking both cals and whether or not something is low or high GL.
Patrick Holford - Low GL Diet Cookbook gives the basic info on GL plus lovely recipes.
I know a lot of people love Rick Gallop's books too.