Similarly, if you've heard that Green Lantern has been well written recently and fancy getting caught up to date, there's no real way to know which books to even buy. Scanning the DC website there's nothing that says "Like this? Read all these other things!" which is, frankly, criminal. Do they not _want_ my money? What I eventually dug up was this - a fucking Amazon List.
I'm sure that if you're a fanboy you know exactly what to read, which message boards will keep you updated, etc. But the casual reader? Lost.
On the other hand, to pick a recent book series - if you want to read all of the Sookie Stackhouse books (the ones True Blood is based on) then a 5 second search lands you at the author's web page, where there's a complete bibliography, complete with ISBN numbers. See, that was simple, wasn't it?