It's the most lively Sam's seen Castiel since they got him back, which he's taking as a good sign. A sign that the research is something that he'd enjoy, which is what Sam wants. Just something to give him a chance to help get himself back together. "Good. It was going to be part of the tour anyway, but we'll come back for something more in depth after we do the shopping." Get Castiel some comfortable clothes. "Once we've done that, I can leave you to familiarizing yourself with the books?"

He knows Dean and he knows his brother is champing at the bit to get back to Jason and to kick his ass. Sam's the same way. More than that, they need to get any of Castiel's money out from under the guy's thumb, though Sam figures that's not likely now. Once Jason realized that Castiel's gone, the bank account was probably closed out. Something they'll deal with later.