G.K. Chesterton is one of the best Christian writers of the twentieth century. Prolific and artistic, he had the knack for combining a classic British commentary sense to any historical Christian subject, making it both the object of cultural interest and often historic reverence. As St. Francis of Assisi was one of the primary influences on Chesterton's decision to convert to Roman Catholicism (Chesterton once described his conversion as being largely due to wanting to belong to the same institution that had produced St. Francis), it makes sense that Chesterton would devote considerable energies toward this biography.
Chesterton said that there are essentially three ways to approach a biography of a figure such as St. Francis - one can be dispassionately objective (or at least as much as can pass for such a stance), looking at things from a 'purely' historical standpoint; one can go to the opposite extreme and treat the figure as an object of devotion and worship; or one can take a third path (and you've guessed correctly if you assumed this was Chesterton's route) of looking at the character as an interested outsider, someone in the modern world but still one involved in the same kinds of structures and virtues as the one being studied.
Chesterton's prose is snappy and lively, witty and bit sardonic at times. Chesterton is not afraid to digress to make his own points, and like the intellectual critic who cannot contain the myriad of responses to particular points, Chesterton treats us to a generous collection of tangential observations. One discovers, for instance, Chesterton's opinion of modern British history (that it reads more like journalism than like a developed narrative) - he makes the observation that journalists rarely think to publish a 'life' until the death of the subject; this of course cannot be helped in the case of Francis of Assisi, but the method of the media serves to highlight the difference in world-view between then and now.
This is a spiritual biography - it does not simply go from event to event in Francis' life, but rather looks as the development of his spirituality, his calling, his order and his influence in later church (and more general) history. In his discussion, he looks at miracles and poetic production, political realities and logical fallacies, ancient sentiments and present-day practices. Francis is seen in many ways as the Mirror of Christ (not quite the same thing as the WWJD fad of the current day, but approximating the sense in some regards), but this sets up an interesting logical situation - if Francis is like Christ, then Christ is in some ways like Francis. Chesterton points out the importance of the difference, likening it to the difference between creator and creature, but there is still the interesting development in history where some tried to make Francis a second Christ (something Francis himself would have opposed bitterly).
Fun, fascinating, spiritual without succumbing to kitsch, intellectual without being overblown, this book is a classic on Francis, and a classic by Chesterton, a small miracle of Francis (in the many sense of the term).