Ros and Guil is the canonical work, and probably the best place to start. Anyone who loves Romantic literature and can listen on their feet could treat themselves to a good production of Arcadia without any fear of disappointment. To the contrary, any contemporary student of Western literature should probably feel compelled to read (or better yet, see) Arcadia.

I've read God Knows and Portrait of the Artist as well, and they're both strong books. Portrait is a quick read, and is exceptionally funny.