Mornings On Horseback: The Story of an Extraordinary Family, a Vanished Way of Life, and the Unique Child Who Became Theodore Roosevelt Right now

I would read the back of a cereal box if David McCullough wrote it so I'm biased to him as an author.
This book should not be the source material for all things T.R. but does fill in the blanks on an interesting family, and goes a long way to explaining some of the details of the man's future life.
The writing here is very good and the material is interesting enough to keep going, even though you are ostensibly reading about the obscure family background of a famous American. Only a commanding figure could even warrant a book about his deep family tree and the details of the homes they lived in and their neighbors.
Good second tier material once you've read the major bios on TR.Get more detail about Mornings On Horseback: The Story of an Extraordinary Family, a Vanished Way of Life, and the Unique Child Who Became Theodore Roosevelt.