I haven't read a lot of Berlin's books. I've read The Power of Ideas, which is a fine collection of essays about Enlightenment philosophers and Russian thinkers. These are his two main areas of interest. Berlin is a historian of ideas fascinated with the philosophical thought of the Enlightenment. He's written excellent essays on Vico and Kant; but he's also interested in Russian writers like Tolstoy and Dostoevsky.
The other book I've read by him is Freedom and its Betrayal, which explores the philosophical thinking of six thinkers he believes were forerunners of Fascism. Amongst them, there's Rousseau. Since I strongly dislike Rousseau, it was impossible not to love a book that basically paints him as a proto-fascist. But even so, the book makes good arguments and provides a clear description of his central ideas. Other philosophers discussed in the book are Fichte, Hegel, Saint-Simon, Maistre and Helvetius. Some of those names may not be very famous, but that's because Berlin really knows what he's talking about and he goes after thinkers who have influenced modern thought even though they've fallen into obscurity. In a way, this is a work of archeology.
It's short, entertaining, and highly informative.