Lyric discussion by PairOFish 

Cover art for Letterbox lyrics by They Might Be Giants

To me this song's about a guy who's in love with a girl and sent her a message admitting his feelings, and now he's anxiously awaiting her reply. He has no idea "what she'll find" (how she'll feel about his love confession)

"'Cause a little bird never tells me anything I want to know, She's my best friend, she's a sparrow"

The "little bird" is the one he loves, who's his best friend but he hopes soon to be girlfriend--she never tells him anything that indicates she loves him, though, which is making him nervous.

"Too late or soon to make noise about love" -- he's not sure if it's too late in their relationship to be more than friends, or if he's being too hasty telling her how he feels. The rest of the song basically just describes his anxiousness (like running around with a hole in his head until he finds out her response tomorrow)