Hellboy simply nods at Powell's recognition of his helpfulness. It didn't really seem like something he was looking for a conversation over, and a more vocal expression of appreciation would've required actively working to make his naturally severe features form his best approximation of a smile. (How any of his friends put up with that, he doesn't know.) So instead he just goes for a solemn nod, and figures that just not scowling will be good enough.

Not trying for expressions also helps mask his mild surprise at how closely they apparently were watching him (and, presumably, others either in the inn, the cohort, or the city in general). "My personal reading time's been a bit limited, but yeah, I'd checked a book out before I moved out. Just returned it, in fact. Haven't decided what I want to read next, yet."