carpentry, repairs and historic house research in Ipswich, MA
Carefully drill holes for very long 1/2" hex-head bolts, then screw the bolts into the wall. Cut the heads of the bolt off and make sure that they are level and square to the wall. Then drill holes in the back of the shelf board and push it on to the bolts.
I used Tiger Claw Hidden Deck Fasteners TC-4 Exotic Hardwoods. With the supplied application tool, you pound the sharp teeth of the fasteners into the side of the deck board, then screws the fastener into the joist.
Gordon Harris 