Kids fiction. I probably read it in the late 1970s. A boy is worried that he’s discovered a spy or crime situation. One memorable scene: he and his grandmother (I believe he lived with her) are talking about it and he’s afraid their room is bugged so he makes them sit under a blanket to talk. There were simple line drawing illustrations which didn’t concern themselves much with actual human proportions.
Stump the Bookseller
New format, same book stumper solving.
One book from the 70s that had the “Is the neighbor a spy?” theme is 51 Sycamore Lane, by Marjorie Sharmat, which was part of the Weekly Reader series. (If there was a chicken involved, it’s almost certainly this one.)
Could this be “The Kidnappers Upstairs” by Eileen Rosenbaum, 1968? Mine was a Scholastic paperback. The illustrations weren’t exactly simple but they were a bit wacky in a very period style. The boy and his grandmother, iirc, live in a NYC apartment and he thinks the spies are living upstairs. They go to various city places like St. Patrick’s Cathedral and maybe a museum; she rides on the back of his bike, which was the cover illustration.