The vast majority of books on photography are actually books about cameras. Or, rather, how to use a camera. And they are, without exception, not interesting. Frankly modern cameras do not require any interference from us to take perfectly sharp, perfectly exposed, perfectly white-balanced images.
Yet precious few of the billions of images that are made daily carry any kind of impact.
On Photography is a meandering, caustic look at what it is to create a photograph that carries meaning.
There is something predatory in the act of taking a picture. To photograph people is to violate them, by seeing them as they never see themselves, by having knowledge of them they can never have; it turns people into objects that can be symbolically possessed.
Photography is not an art, it is a medium, like writing, that can be used to produce art, document events, entertain, lie and steal.
As Wittgenstein argued for words, that the meaning is in the use - so for each photograph.
The taking photographs is a constant part of modern life and, like all common activities, it is not considered deeply (or at all) by those who practice it. This makes Sontag's essays immensely valuable. Sontag doesn't come to any kind of a conclusion, but rather elects to put ideas out there and encourage that rarest of activities: deep consideration.