It is important to distinguish the book (excluding the soft cover book, as it lacks the photos that bring the text to life) from the film.
The film follows in the tracks or at least the intent and aesthetic posturing of much of Jarman's work, from Jubilee to Caravaggio whereas the book fits snugly into a European tradition of deeply personal art theory & criticism.
If you understood that Barthes was really talking about death and love and art was merely a vehicle for him to discuss these topics when he wrote Camera Lucida, or that Jeanette Winterson was following in a uniquely British tradition of personalized academic art criticism when she wrote 'Art Objects' then Jarmen's the last of England is essential.
It traces Jarman's relationship with his father a New Zealander who encounters the wide world for the first time as a pilot of a Lancaster bomber in the second world war. The text details the erosion of this man and his family, through guilt, anxiety (post traumatic depression and stress) due to his involvement in the war and atrocities such as incendentary carpet bombing and the stress which results from the high fatality rates for pilots and aircrews.
Through Jarmans art, writing and particulary the photos it all comes together, a complete weltschauung of Jarmans art and life, and a context not only for Jarman as an emotional human being but also the disintergration of his childhood and familylife and the fall of England after the wars. It is the fall of the British empire on an emotioanal level through the eyes of an artist of some merit.
For myself, I feel that many of his films (excluding blue and Jubilee) are bogged down in theoretical and theatrical masturbation, they seem lost being deeply hip in a gay ghetto; in the book 'The last of England' Jarman accidentally puts forward a work of deep and lasting insight into the human condition, love and loss. I cannot do it justice other than to suggest that books have their fates dependent on the comprehension of the reader.
As I've implied the film is simply out classed by the book and to my mind the reason why Derek Jarman will be remembered as an important British artist, a leading activist for human sexual freedom and a spiritual child of the colonies will in the end be due to the filtering of the emotions and thoughts which came together in the one place, in this book.