This book, originally written in 1935, is, as Ayer says in his introduction, 'very much a young man's book'; that's his way of saying that perhaps he was a little strident at the time. As many of us know, the young tend to believe that everything they stumble across is revealed truth, and it's only after living for many more years that one finds that what they felt they had originally discovered was already known by most other people.
Ayer was one of the most famous logical positivists, had the gift of clear and concise writing and is able to maintain your interest throughout this not-too-long book, especially if you are interested in this sort of thing.
His epiphany? Well, if we refer to Anthony Lewis' characterization of Wittgenstein's early work, we find that 'Language consists of propositions that picture the world; propositions are the perceptible expressions of thoughts, and thoughts are logical pictures of facts'. Therefore, if we analyze propositions, we analyze the 'truth of existence' as it seems to us, anyway. Ayer takes propositions, determines them to be either synthetic or analytic, and judges their truth; i.e. determines reality.
Looking back through the history of Philosophy, the idea of the synthetic and analytic, of the one and the many, of induction and deduction, and of subjective and objective universalism have been fully explored in ancient times, and as (I think) Plato said 'Once a young man discovers the 'One and the Many' he finds it everywhere and there's no stopping him'. It's basically what it's all about, and every 'new' idea in philosophy has been some variant of it. Ayer calls it a 'little book', but I think it's a great book, and a good guide on how to look at things and think about them.