Miss Singerbrains was Jerome Horwitz Elementary School's cruel librarian. Her name is a pun to the phrase "missing her brains."
When she became the librarian, she had the books taken out and/or destroyed and filled the whole library with anti-reading posters. When Melvin in Mr. Krupp's body wants to construct the Purple Potty, he meets Miss Singerbrains in the library. Before he constructs a time machine, he asks her to carry it for him and lies that he'll offer to help her. He then fired Miss Singerbrains (as karma for destroying the books).
Later, when George and Harold reappear from zapping Mr Sneedly with the Forgetchamacallit 2000 and reclaiming the Combine-O-Tron, Miss Singerbrains confiscates them and she decides to take them to the police. Later, George and Harold return, riding on a pterosaur called Crackers and land on top of Miss Singerbrains' car. George tells her this is a dream and when George orders Harold to return Crackers back to the Cretaceous period, he goes in the Purple Potty and transports him to the treehouse. After returning, George leaves with Harold but not before zapping Miss Singerbrains with the Forgetchamacallit. Miss Singerbrains isn't seen for the rest of the book.
Her alternate universe counterpart later has a cameo in the 8th book in which she has not banned the books and is nice and helpful.
She is also seen in the movie In George and Harold's prank put the books hanging on the celling