[pdonis]

> The actual lesson was that you need to be the trueborn king who can claim the palantiri by birthright if you want to use them for good.

Not really. Denethor was the trueborn steward, whose ancestor had been officially appointed by the King, and though it isn't mentioned in the Lord of the Rings, the essay on the Palantiri in Unfinished Tales says that stewards were often deputized to use the Palantiri. So Denethor had the right to use the Palantir of Minas Tirith. But he didn't have the wisdom to realize that Sauron was manipulating what he saw.

[jltsiren]

Denethor is described as wise and strong-willed (unlike the caricature seen in the movies). He knows Sauron is trying to manipulate him, but he believes his strength and legitimacy will let him prevail. It's the same belief Aragorn has, and it's mostly correct. Denethor benefits greatly from the information gained using the palantir. But because his claim is weaker than Aragorn's and because he keeps using it repeatedly over the decades, it eventually corrupts him and leads him to his doom.

[pdonis]

> unlike the caricature seen in the movies

I agree that the movie portrayal was totally unlike the Denethor in the books.

> He knows Sauron is trying to manipulate him

To some extent, yes. But I'm not sure he fully realizes what's going on. For example, he sees the fleet with black sails coming up Anduin--but he didn't see any of the events that led to that fleet being taken over by Aragorn and his followers? He could have.

> because his claim is weaker than Aragorn's

I don't think this is given as a cause of Denethor's doom in the books.

> because he keeps using it repeatedly over the decades

This is mentioned in the books, yes.