I'm in my first year of a photography degree and this is one of the recommended texts. Despite being released in the 70's it's still very relevant.
The book is essentially a series of essays by Sontag. It's value is that Sontag knows her subject extremely well and, with reference to the great photographers, has constructed her essays in a way that really make you contemplate what photography is all about. Sontag was not a photographer herself so it's not a book of technique; it's about the interpretation and appreciation of other's work. If you're new to photographic theory as I was/am it may very well make you feel quite ignorant, as if you don't even know what a photograph really is any more. However, that's the point; to make you think. This most ubiquitous medium is now so common its almost unnoticed but the book gets under the skin of what photography is and why photographs are so appealing. It's not the easiest or lightest reading but conversely the essays are not over long.
If you're studying photography or art or are simply interested in more philosophical views on the subject I'd highly recommend this to open up new ways of thinking.