I've been known to grab total strangers in a bookstore and actually sell that book to them.
The Reagan book is not as compelling as any of the others I've read so far. Perhaps it's that the events are so close in time, and that so many of the characters are real people. He gets into their heads (including Nancy Reagan, as well as former Presidents Carter and Nixon, and many, many other historical personages) and while it feels valid and believable, it's a bit weird.
It does not feel as if he's distorting anything for his own agenda, for what that opinion is worth.
Oh, and Henry and Clara is still in print. So you can buy it, or possibly your library will have it. I can say that I'd never heard of these two before I read the book, and I read the last twenty or so pages with my mouth open in astonishment, as he tells what eventually happened to them.
One of Mallon's strengths is that when he's writing historical characters, he doesn't just write modern people in old clothes. He gives us real (in this case) 19th century people.