The Local Guidebook is entitled 'Jersey & Guernsey' written by Ian Smith and Alan Hill and published by Cordee in 1986 (ISBN 0-904405-39-7). Price is about £6, and available by mail order or from local bookshops. There are also some new routes in the 'BMC New climbs' book from 1987. Since then many other new routes have been put up, so please check with the locals to see whether that unclimbed line has been done.
Alan Hill has also written a book 'Guernsey Coastal Walks and Scrambles'. Published by the Guernsey Press, it is very useful as it gives details of additional access / escape scrambles and other useful information.
I recommend that you get both books if you can, many of the scrambles can be done when it's too wet to climb.
There is no official new routes book, so the GMC would appreciate it if you could email me details of any you do and I'll post them on this site.
If you're interested in climbing in Guernsey,
then email Mark