It is frustrating because it seems to indicate a real push in the publishing world to release books which will become best-sellers or the next Twilight. But this leaves one to wonder where the great stand-alone books will come from in YA? I love books that leave me wanting more and unable to know for certain what happened to the characters after the story ended. The series being produced seem to answer that question all too thoroughly or else lead you to expect a detailed epilogue but only leave you hanging. Susan Beth Pfeffer's Life As We Knew It trilogy left us uncertain to the ultimate fate of her characters when all along it seemed she had brought them together so she could leave us assured of their ultimate fate.
The influx of series are enjoyable precisely because there are some stories which simply put, cannot be restricted to a single volume and others which should have quit after one. For every crappy YA-version of an adult series--I'm looking at you, James Patterson--there are well thought out characters and worlds whose next installation continue to be eagerly waited for. This also opens up more potential for teens to fall in love with a series when they may have previously been 'reluctant readers'. It also produces excellent results in popular genres like fantasy or scifi. I'm a huge fan of the Bloody Jack series in particular but some of the installations have been better than the others, and inevitably they will end and leave us dissatisfied, wanting more of the characters just as if they had been a single-volume novel. So it stands to reason that many authors don't really need to make their story into a series, just maybe a several hundred page work.
What series do you think are excellent and what ones do you think should have stopped after book one?