This is one of the best known books on the cultural context of photography and is quite insightful for the general reader. I would highly recommend it to anyone who wants starting points in attempting to account for the social and cultural significance of photography.
However, I would not recommend it to anyone studying the history of photography at third level, or at least beyond 2nd year. When it comes to deeply researching the subject and its complex role in 20th century culture, Sontag's book has its limits. For research, writers such as Siegfried Kracauer (The Mass Ornament and his many essays on photography) and Walter Benjamin will take you a lot further. The advantage Sontag's book has is that it's an accessible read. While not very simply written, it won't have you running for the dictionary at the same rate that the above-named authors will and is an excellent introduction to the subject. If you want to get to the root of things, go for Kracauer or Benjamin.