Karlin's advice is right on. Our Murphy was a big time resource guarder when we purchased him at a year old. The breeder had taken him back from his first owner because of his issues. He is a very sweet dog but would suddenly decide that an object needed to be guarded with his life! First thing we did was have his thyroid checked and that was OK. Then we worked with a trainer who had us do many excercises with him.
He is 1000% better. There is also a really good book called MINE by Jean Donaldson http://www.dogwise.com/itemdetails.cfm?ID=DTB740 taht is very good! Good luck. PM me if I can help.
Molly and Murphy (tri), Katie (ruby), Casey (B/T),Spike the wonder pup (tri), Suzie (tri) and Penny (blen)