"What happened to love letters?"
Strike says this in the pub when Robin finds him after he finds out Charlotte is engaged. That was in Cuckoo's Calling.
Then at the end of The Silkworm he hands Robin an envelope with the message that if she finds the surveillance course, he'll pay for it. I think that was his first love letter to her. He could've just told her that or even text it to her, but he chose to hand write it and put it in an envelope. Then when she shakes his hand, he holds it and kisses it. Classy move.
I can't get enough of these two, and I'm rooting really hard for them as they slow crawl their way from work colleagues to friends to inevitable lovers.