To use the classic Simpsons meme, the classic multivolume academic biographies of 'the mediocre presidents' - not that Polk really falls into the obscure category, as while his actions were often questionable they certainly are memorable - aren't being written nowadays. As a format, nobody is particularly interested in buying exhaustive examinations of generally unsympathetic characters to a modern audience, so academics tend to stay away from them. One example: what's still the best biography of Warren Harding dates from the late 1960s (and has the portions with his erotic love letters X'd out, since there was a successful restraining order placed on the writer by the Harding family.)

In the case of Polk, the standard reference academic biography is still Charles Sellers' award winning two volume series from the 1950s and 1960s, James K. Polk, Jacksonian/Continentalist, which still holds up well but has never been rereleased digitally so is hard to find.

However, there's been more recent work in shorter but still well researched material. Robert Merry's A Country of Vast Designs is where I'd start. It's a generally positive look at Polk that at least acknowledges his problems with truth and more questionable actions. William Dusinberre's Slavemaster President is a far more critical assessment, and while he shares the stage with several major political and military figures of the Mexican War, Amy Greenburg's A Wicked War offers an outright withering portrayal of Polk's Presidency.

And last, I still have no problems recommending McCollough as a starter biography for his many subjects. They're popular biographies, but they're generally good reads and reasonably well researched. It's when you want to get into more detail and analysis that you go a bit deeper into the lit, and hence why here we recommend things like the two other major biographies of Truman once you've finished him as your introduction.