As you can imagine, the supply of available biographies still in print for Theodore Roosevelt was somewhat larger than the selection for William Howard Taft. Roosevelt was larger than life and such an iconic presence that books are still written about him and make the New York Times best seller list frequently. Taft on the other hand is best known for getting stuck in the bathtub and is largely a foot note in history as an unremarkable president but a better supreme court justice.
The fact that Taft spoke like a supreme court justice and seemed about as likeable as a friendlier Antonin Scalia may have been why he needed a friend to write this biography for him.
The book I read was written in the 1930's by what seems to have been a Taft enthusiast and would certainly have been written when he was still living. My belief is that this was a sort of public opinion minded book written within the recent end of Taft's presidency in 1913, like a "Decision Points" for George W. Bush or "Dreams of My Father" for Barack Obama.
The book explores some interesting parts of Taft's life, like his time where he served as Roosevelt's Secretary of War as the essential ruler of the Philippines after the Spanish American war where it portrays him as a wise and benevolent leader. During and after this time, it also details Taft's desire to join the Supreme Court and details how Roosevelt convinced him instead to "do his duty" and run for the presidency after his term was up in 1908. The author then seems to insinuate Roosevelt craved power and turned on Taft in 1912 where he tried to take the party nomination from him but Taft ultimately prevailed.
More or less, Taft is portrayed as a noble, self sacrificing statesman while Roosevelt is portrayed as a force of nature, not in full control of his own passions and whims.
This is of course the book the author was most likely paid to write, so it's important to take it with a grain of salt, but it's interesting to see an early 20th century biography designed to drive poll numbers more than informing people.