"Stalwart Shield" is not something that can be equipped. It is an advanced fighting style for your Adventuring Fellow that allows it to act more effectively as a tank than the style called simply "Shield".
To be able to use any of the manuals, first you need to complete "Chameleon Capers". Then, to quote wiki:
"To use the manual, talk to your NPC at the Rendezvous Point with it in your inventory. When you talk at the Rendezvous Point again, the new job is available to select from the job style menu."