- Mr. Popper of Mr. Popper's Penguins
- Muriel Ponsonby in Dick King-Smith's The Catlady
- the scarecrow in Philip Pullman's The Scarecrow and His Servant
- the reporter in Michael Morpurgo's The Mozart Question
Wait. Apart from Mr. Popper's Penguins, which was published in 1939, the rest are all published in the UK.
American children's books with adult protagonists, anyone?
When I was a child my father told me stories about Tenali Rama, a character in south Indian folklore who is vastly appealing to children on account of his endearing character flaws as well as his position of combination wise man and jester in the court of the 16th century king Krishna Deva Raya. His lack of real power combines with a few good flaws to create the same odd combination that seems to work for adult protagonists in children's books.
In the YA realm, the emergence of adult protagonists isn't such a huge shift, but may simply reflect a trend toward books that are literally about "young adults" rather than teenagers. Look for The Uninvited by Tim Wynne-Jones, due out next month.