Take a look at the answers posted to the question "Weld Strength", asked by an unidentified person in this same site. The book you have is correct in saying that the material's yield stength for shear is about 40% of the yield strength for tension, compression and flexion (this latter it's me who added). This, however, doesn't preclude you from selecting a suitable safety factor for figuring out the allowable working strength.
Giovanni S. Crisi
Sao Paulo, Brazil