I think a better case can be made that Aragorn's account of events is the second most important source that Tolkien had to draw upon after that of the hobbits themselves. (And Legolas was third in my opinion, though there is less support for this.)

Aragorn is followed by the narrator at Helm's Deep, as I will point out next week. Aragorn is the one followed in Boromir's Departure, and we will follow him into the Last Debate. And Aragorn speaks Rohirric, while Gimli and Legolas do not.

Since in the Prologue's note on Shire Records we see that even the hobbit-written source texts are from Gondor (for the most part), it seems reasonable to guess that other Gondorian texts that detail the exploits of the founding king of the house of Telcontar would be Tolkien's most important secondary texts.

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"You can't even imagine a totally new morality...Try to imagine a society where honesty and justice and courage and self-control and faith and hope and charity are evil, and lying and cheating and stealing and cowardice and betrayal and addiction and despair and hate are all good. You just can't do it."

"Didn't Milton have a line for that? 'Evil be thou my good!'"

"Yes, that was Satan's line, in Paradise Lost. The society we're trying to imagine does exist after all--in hell."