If you are visiting the Island or a Guernsey resident there are a number of tour guides that have regular tours or are happy to arrange a bespoke tour. Their details are on the links below.
I have been on the Batterie Mirus tour with Amanda Johns of Tours of Guernsey and it was excellent. Amanda is very knowledgeable about the occupation of Guernsey and the fortifications. Tours of Guernsey offer a number of tours that you may be interested in. This includes sites not open to the general public. Details on the web page linked above. You can follow Tours of Guernsey here on Twitter @toursofguernsey.
I have also been on a tour with Keith from Guernsey Walking Tours and I have to say he really knows his stuff. He is very knowledgeable about all aspects of Guernsey history. You can follow them on Twitter here @gsywtours.
Tim’s Guided Walks has some great information on his page and he also provides some great guided walks. Take a look here