Those Angry Days is about the struggle to either get the U.S. into World War II, or keep it out, depending on which side of things you fell on. FDR was in the former camp, Charles Lindbergh in the latter, and so they're sort of the two focal characters in the book.
It's not really an equal comparison, because Lindbergh is, for all his fame, a single private citizen, and for much of the debate in the years prior to the U.S.'s entry into the war, he eschews tying himself to an isolationist organization in particular. Of course, while FDR might have very much wanted to the U.S. to lend a hand, he's presented as not doing much to encourage the American public to fall in with him. Even when polls show the public overwhelmingly supports him, he still hesitates to act. Olson attributes this to the drubbing he took when he tried to pack the Supreme Court with judges that would go along with what he wanted, the first time he felt serious resistance from even the people who had supported him all through his New Deal programs. So he never felt as secure in his footing. It's an interesting approach, different from Persico's works that show him as the would-be master manipulator, the man obsessed with keeping his subordinates unsure of what's going on around them, so only he can see the whole picture.
It's also not the only different viewpoint on a figure in the book. Olson doesn't seem to have a very high opinion of George Marshall, describing him as intensely ambitious, and quotes someone calling him a 'consummate Army politician.' Maybe that was meant as a compliment, didn't seem like it, and doesn't jibe with the guy who really badly wanted to command Operation Overlord (Normandy invasion) but wouldn't directly lobby FDR for it, and ended up passing on it because FDR expressed a desire for Marshall to remain in his current position.
As to FDR, based on some things I read about him in that book 1920: The Year of Six Presidents, I think he's just a politician, in the sense of being more than willing to flip-flop on an issue if he thinks it expedient. Also that he knows words are cheap and easy. So he'll declare the U.S. is to become the arsenal of democracy, but not put serious effort into getting factories shifted over to production of armaments. Because that might step on toes.
I'd never read much on Lindbergh, so the book offered a lot I didn't know, especially about the kind of person he was. I did know that he helped the pilots in the Pacific understand how to get better fuel economy out of the P-38 Lightnings, which is how a flight of them were able to intercept Admiral Yamamoto's plane and shoot him down. I didn't know Lindbergh was in the Pacific on the sly, because FDR would never have signed off on it, so friends of Lindbergh's brought him over without telling anyone. For Lindbergh, emotions did not, or were not, supposed to factor into any decision. Everything he said, about the U.S. needing to stay out of the war, or that the British and French were gripped by a lethargy while Germany had an energy and efficiency that got things done, he felt he was saying from a place of rational and logical observation. To him, there was nothing personal in what he was saying, and he couldn't understand why so many people took it that way. If he wrote something, and meant it a certain way, he could not understand how anyone else might draw any other conclusion from it (say that he was advocating open anti-Semitism). Draw from that what you will.
I also didn't know that Anne Lindbergh, originally Anne Morrow, was a multiple-time bestselling author, who unfortunately seemed to have a difficult go of it. Not in the sense of living in poverty, but in feeling especially trapped by everyone else's expectations of who she was supposed to be. Her mother's, her friends, and later, Lindbergh, who expected her to conform to his way of living. Follow where he went, toe his line, don't get emotional.
There's a lot in the book outside of FDR and Lindbergh. There were a lot of moving parts, groups on both sides, with connections in and out of Washington. Shifting loyalties, a presidential election to deal with, pressure from the British to get on with it (without doing so in such a way as to give the isolationists more fuel for their charge that the Brits were trying to trick the U.S. in again).
'In his tenure as president, Roosevelt, who had been so effective in educating Americans about domestic issues, had never done the same for foreign affairs. As his biographer, James McGregor Burns, put it, "He hoped they would be educated by events." As it turned out, they were, but not in the way he wanted.'