When writing a very short introduction, it would seem prudent to keep the intended audience in mind and try to 'introduce' concepts rather than assume too much prior knowledge. The book fails to do either, and reads like one of the dry, overly-academic and dense works I used to have to chew through as a third year cultural studies student; at least I know of some the concepts Flynn is trying to communicate. Many Laymen will not.
Not only is the prose a chore, but the structure is a little scattershot, and there seems no overarching narrative to the historical progress of the field; we swap between Kierkegaarde, Nietzsche, Sartre, Merleau-Ponty, Jaspers and Heidegger in a brainstorm that makes the reader think some of this was a mere distraction for the author. Only the final chapter seems logically positioned.
A number of ideas are not explained, and comparisons with contemporary philosophers who were not existentialists, such as Russell and Wittgenstein, are not there to establish a perspective of where continental existentialism sat with regards to the wider picture of philosophy.
The author does manage to set existentialism against structuralist thinking of later French intellectuals, and addresses the seeming paradox of existentialism's social concerns, which at last gives a vantage point from which to analyse the real-world impact of the movement. There is also a good deal of focus on Sartre, perhaps too much where time could've been given to establishing a better narratiive.
I'm more familiar with the structuralists, so i'd hoped that this book would establish a clear precedent for what i'd learnt about in university, alas it is anything but clear.