It's a blog! Mainly of book reviews.
Reading this feels a bit voyeuristic, in that it was intended as a family document rather than a public one. It's short and not a very good biography; it talks in little detail about Darwin's life or character, whilst rambling about the personalities of various other contemporary scientists, Darwin's religious views and his own books. It's nevertheless of some interest and so short as to hardly allow for getting bogged down. It's nowhere near as fun as The Voyage of the Beagle or as important as On the Origin of Species, however.
It is probably most useful for the section on how developing his theory of evolution eroded his faith in literal interpretation of the Bible and eventually in Christianity altogether.