Destiny's Children, Exultant by Stephen Baxter (2004). Pluto isn't called 'not a planet', but it is called a Kuiper belt object and a worldlet:
There are many bodies out here, some massive. Pluto is one, Port Sol another... But they add up to only about a fifth of Earth's mass. There should have been thousands of worldlets the size of Pluto or larger.
It took the IAU a few years to decide Pluto belonged belonged in the dwarf planet class, but many people, including authors, were aware of the reclassification before it was made official.