I have read some papers by Lewis but never one of his full length books... I thought that I might do so.
I am undecided whether to read Counterfactuals or On the plurality of worlds and wondered if anyone might have a view. OTPW is likely to be more fun I thought but Counterfactuals is perhaps a bit broader?
Ps I am not a pro but not a noob either; I have a philosophy degree and still read serious stuff for recreation and interest.
To a large extent, it depends what you looking for. On the Plurality of Worlds is Lewis' account and defence of modal realism. He thinks possible worlds are not merely hypothetical entities but are as real as our own world or universe is. When we speak of what is actual, we are speaking indexically about our own world, but it has no privileged status among all possible worlds other than being the one we find ourselves in. It is a distinctive and unusual position, but Lewis makes such extensive use of possible worlds in his philosophy, that he considers he is bound to accept it, if only on pragmatic grounds.
Lewis' Counterfactuals was one of the first attempts to set out a logic of counterfactual conditionals. Lewis' contention is that without a proper account of counterfactuals, it is impossible to explain such things as causation, or scientific laws, and that it is something of a scandal that no serious account had previously been given by philosophers. The book is still worth reading, though a great deal has been written about counterfactuals since it was first published in 1973.
However, much of Lewis' most important work is contained within in his papers. If you want to understand his huge influence on modern philosophy then it would be worth reading his collections of published papers, Philosophical Papers, volumes 1 and 2, (Oxford, 1983 and 1986), and the Papers on Philosophical Logic, Metaphysics and Epistemology, and Ethics and Social Philosophy (Cambridge, 1997-99).
I agree with Bumble that it depends on what you're looking for. I initially approached Lewis in a piecemeal way reading his papers and those he cited. If I could do it over, I'd have approached Lewis systematically, trying to understand what he was trying to accomplish overall. Lewis provides a lot of that overall picture in OPW, but it assumes a lot of knowledge of existing debates. I suggest looking at Daniel Nolan's "Lewis" as at least a supplement to any of the reading suggested here. Brian Weatherson provides a good review here: http://brian.weatherson.org/revdldn.pdf
